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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" submissionType="IETF" category="std" consensus="true" docName="draft-ietf-cbor-time-tag-12" number="9581" ipr="trust200902" sortRefs="true" submissionType="IETF" symRefs="true" tocInclude="true" updates="" obsoletes="" xml:lang="en" version="3">

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  <front>
    <title abbrev="CBOR tag Tag for extended time">Concise Extended Time">Concise Binary Object Representation (CBOR) Tags for Time, Duration, and Period</title>
    <seriesInfo name="Internet-Draft" value="draft-ietf-cbor-time-tag-12"/> name="RFC" value="9581"/>
    <author fullname="Carsten Bormann" initials="C." surname="Bormann">
      <organization>Universität Bremen TZI</organization>
      <address>
        <postal>
          <street>Postfach 330440</street>
          <city>Bremen</city>
          <code>D-28359</code>
          <country>Germany</country>
        </postal>
        <phone>+49-421-218-63921</phone>
        <email>cabo@tzi.org</email>
      </address>
    </author>
    <author fullname="Ben Gamari" initials="B." surname="Gamari">
      <organization>Well-Typed</organization>
      <address>
        <postal>
          <street>117 Middle Rd.</street>
          <city>Portsmouth</city>
          <region>NH</region>
          <code>03801</code>
          <country>United States</country> States of America</country>
        </postal>
        <email>ben@well-typed.com</email>
      </address>
    </author>
    <author fullname="Henk Birkholz" initials="H." surname="Birkholz">
      <organization abbrev="Fraunhofer SIT">Fraunhofer Institute for Secure Information Technology</organization>
      <address>
        <postal>
          <street>Rheinstrasse 75</street>
          <city>Darmstadt</city>
          <code>64295</code>
          <country>Germany</country>
        </postal>
        <email>henk.birkholz@sit.fraunhofer.de</email>
        <email>henk.birkholz@ietf.contact</email>
      </address>
    </author>
    <date year="2023"/>
    <keyword>Internet-Draft</keyword> year="2024" month="August"/>
    <area>art</area>
    <workgroup>cbor</workgroup>

<keyword>Point in time</keyword>
<keyword>Duration</keyword>
<keyword>Period in time</keyword>
<keyword>Clock accuracy</keyword>
<keyword>Timescale</keyword>
<keyword>TAI</keyword>
<keyword>UTC</keyword>
<keyword>NTP</keyword>
<keyword>Network Time Protocol</keyword>
<keyword>PTP</keyword>
<keyword>Precision Time Protocol</keyword>

    <abstract>
      <?line 167?>

<t>The Concise Binary Object Representation (CBOR, RFC 8949) is a data
format whose design goals include the possibility of extremely small
code size, fairly small message size, and extensibility without the
need for version negotiation.</t>
      <t>In CBOR, one point of extensibility is the definition of CBOR tags.
RFC 8949 defines two tags for time: CBOR tag 0 (RFC3339 (RFC 3339 time as a string) and tag
1 (POSIX time as int or float).  Since then, additional requirements have
become known.  The present document defines a CBOR tag for time that
allows a more elaborate representation of time, as well as related
CBOR tags for duration and time period.  This document is
intended as the reference document for the IANA registration of the
CBOR tags defined.</t>
      <t><cref anchor="status">(This cref will be removed by the RFC editor:)<br/>
The present revision (–12) addresses the IESG reviews.</cref></t>
    </abstract>
    <note removeInRFC="true">
      <name>About This Document</name>
      <t>
        Status information for this document may be found at <eref target="https://datatracker.ietf.org/doc/draft-ietf-cbor-time-tag/"/>.
      </t>
      <t>
        Discussion of this document takes place on the
        CBOR Working Group mailing list (<eref target="mailto:cbor@ietf.org"/>),
        which is archived at <eref target="https://mailarchive.ietf.org/arch/browse/cbor/"/>.
        Subscribe at <eref target="https://www.ietf.org/mailman/listinfo/cbor/"/>.
      </t>
      <t>Source for this draft and an issue tracker can be found at
        <eref target="https://github.com/cbor-wg/time-tag"/>.</t>
    </note>
  </front>
  <middle>
    <?line 191?>

<section anchor="intro">
      <name>Introduction</name>
      <t>The Concise Binary Object Representation (CBOR, (CBOR) <xref target="RFC8949"/>) target="RFC8949"/> provides
for the interchange of structured data without a requirement for a
pre-agreed schema.
RFC 8949 defines a basic set of data types, as well as a tagging
mechanism that enables extending the set of data types supported via
an IANA registry for CBOR tags (<xref (see <xref target="IANA.cbor-tags"/> and <xref section="9.2" sectionFormat="of" target="RFC8949"/>, <xref target="IANA.cbor-tags"/>).</t> target="RFC8949"/>).</t>
      <t>RFC 8949 defines two tags for time: CBOR tag 0 (RFC3339 (<xref target="RFC3339"/> time as a string) and tag
1 (POSIX time as int or float).  Since then, additional requirements have
become known.  The present document defines a CBOR tag for time that
allows a more elaborate representation of time, as well as related
CBOR tags for duration durations and time period. periods.  This document is
intended as the reference document for the IANA registration of the
CBOR tags defined.</t>
      <section anchor="terms">
        <name>Terminology</name>
        <t>The
        <t>
    The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
    "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>",
    "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>",
    "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are to be
    interpreted as described in BCP 14 BCP&nbsp;14 <xref target="RFC2119"/> <xref
    target="RFC8174"/> when, and only when, they appear in all capitals, as
    shown here.</t> here.
        </t>
        <?line -18?>

<t>The term "byte" is used in its now customary sense as a synonym for
"octet".</t>
        <t>Superscript notation denotes exponentiation.  For example, 2 to the
power of 64 is notated: 2<sup>64</sup>.  In the plain-text rendition
of this specification, superscript notation is not available and
exponentiation therefore is therefore rendered by the surrogate notation seen
here in the plain-text rendition.</t>
        <t>CBOR diagnostic notation is defined in <xref section="8" sectionFormat="of" target="RFC8949"/> and
<xref section="G" sectionFormat="of" target="RFC8610"/>.
A machine-processable model of the data structures defined in this
specification is provided throughout the text using the Concise Data
Definition Language, CDDL Language (CDDL) <xref target="RFC8610"/>; <xref target="collected-cddl"/> provides the
collected model information.</t>
        <t>Several time-related terms terms, such as UTC and TAI International Atomic Time (TAI), are discussed in
<xref target="IXDTF"/>, target="RFC9557"/>, which may be a useful companion document beyond its direct
use in Sections <xref format="counter" target="tzh"/> and <xref format="counter" target="suff"/>.</t>
      </section>
    </section>
    <section anchor="objectives">
      <name>Objectives</name>
      <t>For the time tag,
the present specification addresses the following objectives that go
beyond the original tags 0 and 1 (defined in Sections <xref section="3.4.1" sectionFormat="bare" target="RFC8949"/> and <xref section="3.4.2" sectionFormat="bare" target="RFC8949"/> of <xref target="RFC8949"/>):</t>
      <ul spacing="normal">
        <li>
	  <t>Additional resolution for epoch-based time (as in tag 1).  CBOR tag 1 only provides for representation of time as an integer and as up to a binary64 floating point
representation of times, limiting floating-point value <xref target="IEEE754"/>, which limits the resolution to approximately
microseconds at the time of writing (and progressively (progressively becoming
worse over time).</t>
        </li>
        <li>
          <t>Indication of timescale.  Tags 0 and 1 are defined for UTC; however, some
interchanges are better performed on TAI.  Other timescales may be
registered once they become relevant (e.g., one of the proposed
successors to UTC that might no longer use leap seconds, seconds or a
scale based on smeared leap seconds).</t>
        </li>
      </ul>

      <t>By incorporating a way to transport <xref target="IXDTF"/> target="RFC9557"/> suffix information (<xref target="tzh"/>, (see Sections <xref target="suff"/>), target="tzh" format="counter"/> and
<xref target="suff" format="counter"/>), additional indications can be provided of intents about the
interpretation of the time given, given can be provided; in particular also particular, for instances of
time that, at the time they are being described, are in the future.
Intents might include information about time zones, daylight savings saving
times, preferred calendar representations, etc.</t>
      <t>Semantics not covered by this document can be added by registering
additional map keys for the map that is the content of the tag (see
<tt>etime-detailed</tt> in <xref target="tag-1001"/>),
the specification for
which is referenced by the registry entry (see <xref target="time-format"/>).</t>
      <t>For example, map keys could be registered for direct representations
of natural platform time formats.  Some platforms use epoch-based
time formats that require some computation to convert them into the
representations allowed by tag 1; these computations can also lose
precision and cause ambiguities.  (The present specification does
not take a position on whether tag 1 can be "fixed" to include,
e.g., Decimal or BigFloat Bigfloat representations.  It does define how to
use these representations with the extended time format.)</t>
      <t>Additional tags are defined for durations and periods.</t>
    </section>
    <section anchor="time-format">
      <name>Time Format</name>
      <t>An extended time is indicated by CBOR tag 1001, the content of which is a map data
item (CBOR major type 5).  The map may contain integer (major types 0
and 1) or text string (major type 3) keys, with the value type
determined by each specific key.
For negative integer keys and text string values of the key,
implementations <bcp14>MUST</bcp14> ignore key/value pairs they do not understand;
these keys are "elective", as the extended time as a whole is still
usable without the information they carry if an implementation elects not to implement them. Conversely, for unsigned integer keys, implementations <bcp14>MUST</bcp14> signal as an error
when encountering key/value pairs that use unsigned integer keys they do not understand or implement (these
are either "base time" or "critical", see below).</t>
      <t>The map <bcp14>MUST</bcp14> contain exactly one unsigned integer key that specifies
the "base time", time" and <bcp14>MAY</bcp14> also contain one or more negative integer or
text-string keys, which may encode supplementary information.</t>
      <t>Supplementary information <bcp14>MAY</bcp14> also be provided by additional unsigned
integer keys that are explicitly defined to provide supplementary
information (we say these keys are defined to be "critical"); as these
are required to be understood, there can be no confusion with base
time keys.</t>
      <t>Negative integer and text string keys always supply supplementary
information (they are "elective", and this will not be explicitly stated
below).</t>
      <t>Supplementary information may include:</t>
      <ul spacing="normal">
        <li>
          <t>a higher precision time offset to be added to the base time,</t>
        </li>
        <li>
          <t>a reference timescale and epoch different from the default UTC and 1970-01-01</t> 1970-01-01, and</t>
        </li>
        <li>
          <t>information about clock quality parameters, such as source,
accuracy, and uncertainty</t> uncertainty.</t>
        </li>
      </ul>
      <t>Additional keys can be defined by registering them in the "Time Tag Map Key
Registry Keys"
registry (<xref target="map-key-registry"/>).
Registered keys may, for instance, add intent information such as timezone and time zone, daylight savings saving time,
and/or possibly positioning coordinates, coordinates to express information that would indicate a local time.</t>
      <t>This document does not define supplementary text keys.
A number of both unsigned and negative-integer keys are defined in
the following subsections.</t>
      <t><xref target="tag-1001"/> provides a formal definition of Tag tag 1001 in CDDL.</t>
      <figure anchor="tag-1001">
        <name>CDDL definition Definition of Tag 1001</name>
        <sourcecode type="cddl"><![CDATA[
Etime = #6.1001(etime-detailed)

etime-framework = {
  uint => any ; at least one base time
  * (nint/text) => any ; elective supplementary information
  * uint => any ; critical supplementary information
}

etime-detailed = ({
  $$ETIME-BASETIME
  ClockQuality-group
  * $$ETIME-ELECTIVE
  * $$ETIME-CRITICAL
  * ((nint/text) .feature "etime-elective-extension") => any
  * (uint .feature "etime-critical-extension") => any
}) .within etime-framework
]]></sourcecode>
      </figure>
      <section anchor="key-1">
        <name>Key 1</name>
        <t>Key 1 indicates a base time value that is exactly like the data item that would
be tagged by CBOR tag 1 (POSIX time <xref target="TIME_T"/> as int or float).
As described above, the time value indicated by the value under this
key can be further
modified by other keys.</t>
        <sourcecode type="cddl"><![CDATA[
$$ETIME-BASETIME //= (1: ~time)
]]></sourcecode>
      </section>
      <section anchor="keys-4-and-5">
        <name>Keys 4 and 5</name>
        <t>Keys 4 and 5 indicate a base time value and are like key 1, except that the data item is an array as
defined for CBOR tag 4 or 5, respectively.
This can be used to include a Decimal or Bigfloat epoch-based float
<xref target="TIME_T"/> in an extended time, e.g., to achieve higher resolution or to
avoid rounding errors.</t>
        <sourcecode type="cddl"><![CDATA[
$$ETIME-BASETIME //= (4: ~decfrac)
$$ETIME-BASETIME //= (5: ~bigfloat)
]]></sourcecode>
      </section>
      <section anchor="keys-3-6-9-12-15-18">
        <name>Keys -3, -6, -9, -12, -15, and -18</name>
        <t>The keys -3, -6, -9, -12, -15 -15, and -18 indicate additional decimal fractions by
giving an unsigned integer (major type 0) and scaling this with the
scale factor 1e-3, 1e-6, 1e-9, 1e-12, 1e-15, and 1e-18, respectively
(see <xref target="decfract"/>).
Each extended time data item <bcp14>MUST NOT</bcp14> contain more than one
of these keys.
These additional fractions are added to a base time in seconds <xref target="SI-SECOND"/>
indicated by a Key key 1, which then <bcp14>MUST</bcp14> also be present and <bcp14>MUST</bcp14> have an
integer value.</t>
        <table anchor="decfract">
          <name>Key
          <name>Keys for decimally scaled Decimally Scaled Fractions</name>
          <thead>
            <tr>
              <th align="left">Key</th>
              <th align="left">meaning</th> align="left">Meaning</th>
              <th align="left">example usage</th> align="left">Example Usage</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td align="left">-3</td>
              <td align="left">milliseconds</td>
              <td align="left">Java time</td>
            </tr>
            <tr>
              <td align="left">-6</td>
              <td align="left">microseconds</td>
              <td align="left">(old) UNIX time</td>
            </tr>
            <tr>
              <td align="left">-9</td>
              <td align="left">nanoseconds</td>
              <td align="left">(new) UNIX time</td>
            </tr>
            <tr>
              <td align="left">-12</td>
              <td align="left">picoseconds</td>
              <td align="left">Haskell time</td>
            </tr>
            <tr>
              <td align="left">-15</td>
              <td align="left">femtoseconds</td>
              <td align="left">(future)</td>
            </tr>
            <tr>
              <td align="left">-18</td>
              <td align="left">attoseconds</td>
              <td align="left">(future)</td>
            </tr>
          </tbody>
        </table>
        <sourcecode type="cddl"><![CDATA[
$$ETIME-ELECTIVE //= (-3: uint)
$$ETIME-ELECTIVE //= (-6: uint)
$$ETIME-ELECTIVE //= (-9: uint)
$$ETIME-ELECTIVE //= (-12: uint)
$$ETIME-ELECTIVE //= (-15: uint)
$$ETIME-ELECTIVE //= (-18: uint)
]]></sourcecode>
        <t>Note that these keys have been provided to facilitate representing
pairs of the form second/decimal fraction of a second, as found for
instance in C <tt>timespec</tt> (Section 7.27.1 of <xref target="C"/>).
When ingesting a timestamp with one of these keys into a type provided
by the target platform, care has to be taken to meet its invariants.
E.g.,
For example, for C <tt>timespec</tt>, the fractional part <tt>tv_nsec</tt> needs to be
between 0 inclusive and 10<sup>9</sup> exclusive, which can be
achieved by also adjusting the base time appropriately.</t>
      </section>
      <section anchor="key-timescale">
        <name>Key -1:
        <name>Keys -1, -13, and 13: Timescale</name>
        <t>Key -1 is
        <t>Keys -1, -13, and 13 are used to indicate a timescale.  The timescale, where key 13 is critical. Keys -1 and -13 have identical semantics (both are assigned because
 key -1 was chosen first and then, when key 13 was added, it appeared desirable to have a
 negative equivalent).
 Each extended time data item <bcp14>MUST NOT</bcp14>
	contain more than one of these keys.</t>
	<t>The value 0 indicates UTC,
with the POSIX epoch <xref target="TIME_T"/>; the value 1 indicates TAI, with the
PTP (Precision
Precision Time Protocol) Protocol (PTP) epoch (1 January 1970 00:00:00 TAI, see
<xref target="IEEE1588-2019"/> or <xref target="IEEE1588-2008"/>).</t>
        <sourcecode type="cddl"><![CDATA[
$$ETIME-ELECTIVE //= (-1 => $ETIME-TIMESCALE)
$$ETIME-ELECTIVE //= (-13 => $ETIME-TIMESCALE)
$$ETIME-CRITICAL //= (13 => $ETIME-TIMESCALE)

$ETIME-TIMESCALE /= &(etime-utc: 0)
$ETIME-TIMESCALE /= &(etime-tai: 1)
]]></sourcecode>
        <t>If key -1 is not none of the keys are present, the default timescale value 0 is implied.</t>
        <t>Timescale values <bcp14>MUST</bcp14> be unsigned integers or text strings; text strings are
provided for experimentation and <bcp14>MUST NOT</bcp14> be used between parties
which
that are not both part of the experiment.
Additional unsigned integer values can be registered in the Timescale Registry "Timescales" registry
(<xref target="timescale-registry"/>).
(Note that there should be no timescales "GPS" or "NTP" <xref target="RFC5905"/>  -- instead,
the time should be converted to TAI or UTC using a single addition or subtraction.)</t>
        <figure anchor="offset">
          <name>Converting Common Offset Timescales</name>
          <artset>
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            </artwork>
            <artwork type="ascii-art"><![CDATA[t    = t    - 2208988800
 utc    ntp

t    = t    + 315964819
 tai    gps
]]></artwork>
          </artset>
        </figure>
        <aside>
          <t>Editor's note:
This initial set of timescales was deliberately chosen to be frugal, as
the specification of the tag provides an extension point where
additional timescales can be registered at any time.
Registrations are clearly needed for earth-referenced timescales (such
as UT1 and TT), as well as possibly for specific realizations of
abstract time scales timescales (such as TAI(USNO) TAI(USNO), the specific realization obtained at the United States Naval Observatory, which is more accurate as a
constant offset basis for GPS times).
While the registration process itself is trivial, these registrations
need to be made based on a solid specification of their actual
definition.</t>
        </aside>
      </section>
      <section anchor="clock-quality">
        <name>Clock Quality</name>
        <t>A number of keys are defined to indicate the quality of the clock that was
used to determine the point in time.</t>
        <t>The first three are analogous to <tt>clock-quality-grouping</tt> in
<xref target="RFC8575"/>, which is in turn based on the definitions in
<xref target="IEEE1588-2008"/>; the last two are specific to this document.</t>
        <sourcecode type="cddl"><![CDATA[
ClockQuality-group = (
  ? &(ClockClass: -2) => uint .size 1 ; PTP/RFC8575
  ? &(ClockAccuracy: -4) => uint .size 1 ; PTP/RFC8575
  ? &(OffsetScaledLogVariance: -5) => uint .size 2 ; PTP/RFC8575
  ? &(Uncertainty: -7) => ~time/~duration
  ? &(Guarantee: -8) => ~time/~duration
)
]]></sourcecode>
        <section anchor="clockclass-key-2">
          <name>ClockClass (Key -2)</name>
          <t>Key -2 (ClockClass) can be used to indicate the clock class as per
<xref target="RFC8575"/> (which is based on Table 5 in Section 7.6.2.4 of <xref target="IEEE1588-2008"/>; Table 4 in Section 7.6.2.5 of <xref target="IEEE1588-2019"/> has updated language).
It is defined as a one-byte unsigned integer as that is the range
defined in IEEE 1588.</t>
        </section>
        <section anchor="clockaccuracy-key-4">
          <name>ClockAccuracy (Key -4)</name>
          <t>Key -4 (ClockAccuracy) can be used to indicate the clock accuracy as
per <xref target="RFC8575"/> (which is based on Table 6 in Section 7.6.2.5 of <xref target="IEEE1588-2008"/>; additional values have been defined in Table 5 in
Section 7.6.2.6 of <xref target="IEEE1588-2019"/>).
It is defined as a one-byte unsigned integer as that is the range defined there.
The range between 23 and 47 is a slightly distorted logarithmic scale
from 1 ps 1 ps to 1 s 1 s in <xref target="IEEE1588-2019"/> (in <xref target="IEEE1588-2008"/> target="IEEE1588-2008"/>, the range was
a subset of that, 32 to 47 for 25 ns 25 ns to 1 s) — 1 s) -- see
<xref target="formula-accuracy-enum"/>; the number 254 is the
value to be used if an unknown accuracy needs to be expressed.</t>
          <!-- Note that the double space after the \approx is actually needed -->
<!-- by utftex -->
<figure anchor="formula-accuracy-enum">
            <name>Approximate conversion Conversion from accuracy Accuracy to accuracy enumeration value</name> Accuracy Enumeration Value</name>
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                      <use xlink:href="#E1-STIXWEBNORMALI-1D45A-gensym002" x="0" y="0"/>
                      <g transform="translate(710,-150)">
                        <use xlink:href="#E1-STIXWEBNORMALI-1D44E-gensym002" transform="scale(0.707)" x="0" y="0"/>
                        <use xlink:href="#E1-STIXWEBNORMALI-1D450-gensym002" transform="scale(0.707)" x="502" y="0"/>
                        <use xlink:href="#E1-STIXWEBNORMALI-1D450-gensym002" transform="scale(0.707)" x="918" y="0"/>
                      </g>
                    </g>
                    <use xlink:href="#E1-STIXWEBMAIN-2248-gensym002" x="3448" y="0"/>
                    <g transform="translate(4411,0)">
                      <use xlink:href="#E1-STIXWEBMAIN-34-gensym002"/>
                      <use xlink:href="#E1-STIXWEBMAIN-38-gensym002" x="500" y="0"/>
                    </g>
                    <use xlink:href="#E1-STIXWEBMAIN-2B-gensym002" x="5635" y="0"/>
                    <use xlink:href="#E1-STIXWEBMAIN-230A-gensym002" x="6542" y="0"/>
                    <use xlink:href="#E1-STIXWEBMAIN-32-gensym002" x="7012" y="0"/>
                    <use xlink:href="#E1-STIXWEBMAIN-22C5-gensym002" x="7735" y="0"/>
                    <use xlink:href="#E1-STIXWEBNORMALI-1D459-gensym002" x="8243" y="0"/>
                    <use xlink:href="#E1-STIXWEBNORMALI-1D45C-gensym002" x="8562" y="0"/>
                    <g transform="translate(9020,0)">
                      <use xlink:href="#E1-STIXWEBNORMALI-1D454-gensym002" x="0" y="0"/>
                      <g transform="translate(492,-150)">
                        <use xlink:href="#E1-STIXWEBMAIN-31-gensym002" transform="scale(0.707)"/>
                        <use xlink:href="#E1-STIXWEBMAIN-30-gensym002" transform="scale(0.707)" x="500" y="0"/>
                      </g>
                    </g>
                    <g transform="translate(10321,0)">
                      <g transform="translate(120,0)">
                        <rect height="60" width="1453" x="0" y="220"/>
                        <g transform="translate(60,676)">
                          <use xlink:href="#E1-STIXWEBNORMALI-1D44E-gensym002" x="0" y="0"/>
                          <use xlink:href="#E1-STIXWEBNORMALI-1D450-gensym002" x="502" y="0"/>
                          <use xlink:href="#E1-STIXWEBNORMALI-1D450-gensym002" x="918" y="0"/>
                        </g>
                        <use xlink:href="#E1-STIXWEBMAIN-73-gensym002" x="532" y="-686"/>
                      </g>
                    </g>
                    <use xlink:href="#E1-STIXWEBMAIN-2212-gensym002" x="12236" y="0"/>
                    <use xlink:href="#E1-STIXWEBNORMALI-1D700-gensym002" x="13144" y="0"/>
                    <use xlink:href="#E1-STIXWEBMAIN-230B-gensym002" x="13629" y="0"/>
                  </g>
                </svg>
              </artwork>
              <artwork type="ascii-art"><![CDATA[                         acc
enum    ≈ 48 + ⌊2 ⋅log   ──── - ε⌋
    acc               10  s
]]></artwork>
            </artset>
          </figure>
        </section>
        <section anchor="offsetscaledlogvariance-key-5">
          <name>OffsetScaledLogVariance (Key -5)</name>
          <t>Key -5 (OffsetScaledLogVariance) can be used to represent the variance
exhibited by the clock when it has lost its synchronization with an
external reference clock.  The details for the computation of this
characteristic are defined in Section 7.6.3 of <xref target="IEEE1588-2019"/> and the
same section in <xref target="IEEE1588-2008"/>.</t>
        </section>
        <section anchor="uncertainty-key-7">
          <name>Uncertainty (Key -7)</name>

          <t>Key -7 (Uncertainty) can be used to represent a known measurement uncertainty of measurement for the clock, clock as a numeric value in seconds or as a
	  duration (<xref target="duration"/>).</t>
          <t>For this document, uncertainty is defined as in Section 2.2.3 of <xref target="GUM"/>: "parameter, associated with the result of a measurement, that
characterizes the dispersion of the values that could reasonably be
attributed to the measurand".  More specifically, the value for this
key represents the expanded uncertainty for k = 2 k = 2 (Section 6.2.1 of <xref target="GUM"/>), target="GUM"/>) in seconds.</t>
          <t>Note that the additional information that can be meaningfully provided
with the duration that represents an uncertainty is limited, e.g., it
is not customary to provide an uncertainty for a duration representing
an uncertainty.
Implementations are free to reduce the information contained in an
uncertainty (which is already elective) to the information they can
process.</t>
          <t>For example, a timestamp that is given to a resolution of
10<sup>-6</sup> seconds (microseconds) but only has an uncertainty of
10<sup>-3</sup> seconds (milliseconds) could be expressed by one of
the extended time tags in <xref target="uncertainty-example"/> (note the slight
rounding error in the third case, which is probably inconsequential
for an uncertainty value):</t>
          <figure anchor="uncertainty-example">
            <name>Examples Using Uncertainty</name>
            <sourcecode type="cbor-diag"><![CDATA[
1001({1: 1697724754, -6: 873294, -7: {1: 0, -6: 1000}}),
1001({1: 1697724754, -6: 873294, -7: {1: 0, -3: 1}}),
1001({1: 1697724754, -6: 873294, -7: {1: 0.001}})
]]></sourcecode>
          </figure>
        </section>
        <section anchor="guarantee-key-8">
          <name>Guarantee (Key -8)</name>
          <t>Key -8 (Guarantee) can be used to represent a stated guarantee for the
accuracy of the point in time, time as a numeric value in seconds or as a
duration (<xref target="duration"/>)
representing the maximum allowed deviation from the true value.</t>
          <t>While such a guarantee is unattainable in theory, existing standards
such as <xref target="RFC3161"/> stipulate the representation of such guarantees,
and therefore this format provides a way to represent them as well;
the time value given is nominally guaranteed to not deviate from the
actual time by more than the value of the guarantee, guarantee in seconds.</t>
          <t>Note that the additional information that can be meaningfully provided
with the duration that represents a guarantee is limited, e.g., it is
not meaningful to provide a guarantee of accuracy for the duration
representing a guarantee of accuracy.
Implementations are free to reduce a guarantee (which is already
elective) to the information they can process.</t>
        </section>
      </section>
      <section anchor="tzh">
        <name>Keys -10, 10: Time Zone Hint</name>
        <t>Keys -10 and 10 supply supplementary information, where key 10 is critical.</t>
        <t>They can be used to provide a hint about the time zone that
would best fit for displaying the time given to humans, using a text
string in the format defined for <tt>time-zone-name</tt> or <tt>time-numoffset</tt>
in <xref target="IXDTF"/>. target="RFC9557"/>.
Key -10 is equivalent to providing this information as an elective
hint, while key 10 provides this information as critical (i.e., it
<bcp14>MUST</bcp14> be used when interpreting the entry with this key).</t>
        <t>Keys -10 and 10 <bcp14>MUST NOT</bcp14> both be present.</t>

        <sourcecode type="cddl"><![CDATA[
$$ETIME-ELECTIVE //= (-10: time-zone-info)
$$ETIME-CRITICAL //= (10: time-zone-info)

time-zone-info = tstr .abnf
                 ("time-zone-name / time-numoffset" .det IXDTFtz)
IXDTFtz = '
   time-hour       = 2DIGIT  ; 00-23
   time-minute     = 2DIGIT  ; 00-59
   time-numoffset  = ("+" / "-") time-hour ":" time-minute

   time-zone-initial = ALPHA / "." / "_"
   time-zone-char    = time-zone-initial / DIGIT / "-" / "+"
   time-zone-part    = time-zone-initial *13(time-zone-char) *time-zone-char
                       ; but not "." or ".."
   time-zone-name    = time-zone-part *("/" time-zone-part)
   ALPHA             =  %x41-5A / %x61-7A   ; A-Z / a-z
   DIGIT             =  %x30-39 ; 0-9
' ; extracted from [IXDTF] [RFC9557] and [RFC3339]; update as needed [RFC3339]
]]></sourcecode>
      </section>
      <section anchor="suff">
        <name>Keys -11, 11: IXDTF Suffix Information</name>
        <t>Keys -11 and 11 supply supplementary information, where key 11 is critical.</t>
        <t>Similar to keys -10 and 10, keys -11 (elective) and 11 (critical) can
be used to provide additional information in the style of IXDTF Internet Extended Date/Time Format (IXDTF)
suffixes, such as the calendar that would best fit for displaying the
time given to humans.
The key's value is a map that has IXDTF <tt>suffix-key</tt> names as keys and
	corresponding suffix values as values, specifically:</t>

        <sourcecode type="cddl"><![CDATA[
$$ETIME-ELECTIVE //= (-11: suffix-info-map)
$$ETIME-CRITICAL //= (11: suffix-info-map)

suffix-info-map = { * suffix-key => suffix-values }
suffix-key = tstr .abnf ("suffix-key" .det IXDTF)
suffix-values = one-or-more<suffix-value>
one-or-more<T> = T / [ 2* T ]
suffix-value = tstr .abnf ("suffix-value" .det IXDTF)

IXDTF = '
   key-initial       = lcalpha / "_"
   key-char          = key-initial / DIGIT / "-"
   suffix-key        = key-initial *key-char

   suffix-value      = 1*alphanum
   alphanum          = ALPHA / DIGIT
   lcalpha           =  %x61-7A
   ALPHA             =  %x41-5A / %x61-7A   ; A-Z / a-z
   DIGIT             =  %x30-39 ; 0-9
' ; extracted from [IXDTF]; update as needed! [RFC9557]
]]></sourcecode>
        <t>When keys -11 and 11 both are both present, the two maps <bcp14>MUST NOT</bcp14> have
entries with the same map keys.</t>
        <t>Figure 4 of <xref target="IXDTF"/> target="RFC9557"/> gives an example for an extended date-time with both time zone
	and suffix information:</t>
        <artwork><![CDATA[
1996-12-19T16:39:57-08:00[America/Los_Angeles][u-ca=hebrew]
]]></artwork>
        <t>A time tag that is approximating this example, in CBOR diagnostic
notation, would be:</t>
        <sourcecode type="cbor-diag"><![CDATA[
/ 1996-12-19T16:39:57-08:00[America//Los_Angeles][u-ca=hebrew] /
1001({ 1: 851042397,
     -10: "America/Los_Angeles",
     -11: { "u-ca": "hebrew" }
})
]]></sourcecode>
        <t>Note that both -10 and -11 are using negative keys and therefore
provide elective information, as in the IXDTF form given in the comment.
Note also that
Also note that, in this example example, the time numeric offset (<tt>-08:00</tt>) is
lost in translating from the <xref target="RFC3339"/> information in the IXDTF into a
POSIX time that can be included under Key key 1 in a time tag.</t>
      </section>
    </section>
    <section anchor="duration">
      <name>Duration Format</name>
      <t>A duration is the length of an interval of time.
Durations in this format are given in SI International System of Units (SI) seconds, possibly adjusted
for conventional corrections of the timescale given (e.g., leap
seconds).</t>
      <t>Except for using Tag tag 1002 instead of 1001,
durations are structurally identical to time values.</t>
      <sourcecode type="cddl"><![CDATA[
Duration = #6.1002(etime-detailed)
]]></sourcecode>
      <t>Semantically, they do not measure the time elapsed from a given epoch, epoch
but from the start to the end of (an otherwise an (otherwise unspecified) interval
of time.</t>
      <t>In combination with an epoch identified in the context, a duration can
also be used to express an absolute time.</t>
      <t>Without such context, durations are subject to some uncertainties
underlying the timescale used.
E.g.,
For example, for durations intended as a determinant of future time periods,
there is some uncertainty of what irregularities (such as leap
seconds,
seconds and timescale corrections) will be exhibited by the timescale in
that period.
For durations as measurements of past periods, abstracting the period
to a duration loses some detail about timescale irregularities.
For many applications, these uncertainties are acceptable and thus
the use of durations is appropriate.</t>
      <aside>
        <t>Note that the durations defined in <xref target="ISO8601"/> and <xref target="ISO8601-2019"/> are rather different from the ones defined
in the present specification; there is no intention to support ISO 8601
durations here.</t>
      </aside>
    </section>
    <section anchor="period">
      <name>Period Format</name>
      <t>A period is a specific interval of time, specified as either two
extended times giving the start and the end of that interval, interval or as
one of these two plus a duration.</t>
      <t>This is represented as an array of unwrapped time and duration elements,
tagged with Tag tag 1003, one of:</t>
      <ul spacing="normal">
        <li>
          <t>a start and end time, in which case the
tag content is an array of two unwrapped extended time elements;</t> elements, or</t>
        </li>
        <li>
          <t>a start time with duration or an end time with duration.
The tag content is an array of 3 elements: the first two as
above but either the start or end time <bcp14>MUST</bcp14> be set to null; null and the
third one then is an unwrapped duration.</t>
        </li>
      </ul>
      <t>A simple CDDL definition that does not capture all the constraints is:</t>
      <sourcecode type="cddl"><![CDATA[
simple-Period = #6.1003([
  start: ~Etime / null
  end: ~Etime / null
  ? duration: ~Duration
])
]]></sourcecode>
      <t>Exactly two out of the three elements must be present and non-null;
this can be somewhat more verbosely expressed in CDDL as:</t>
      <sourcecode type="cddl"><![CDATA[
Period = #6.1003([
  (start: ~Etime,
   ((end: ~Etime) //
    (end: null,
     duration: ~Duration))) //
  (start: null,
   end: ~Etime,
   duration: ~Duration)
])
]]></sourcecode>
    </section>
    <section anchor="cddl-typenames">
      <name>CDDL typenames</name> Type Names</name>
      <t>When detailed validation is not needed, the
type names defined in <xref target="tag-cddl"/> are recommended:</t>
      <figure anchor="tag-cddl">
        <name>Recommended type names Type Names for CDDL</name>
        <sourcecode type="cddl"><![CDATA[
etime = #6.1001({* (int/tstr) => any})
duration = #6.1002({* (int/tstr) => any})
period = #6.1003([~etime/null, ~etime/null, ~duration/null]) ?~duration])
]]></sourcecode>
      </figure>
    </section>
    <section anchor="iana-considerations">
      <name>IANA Considerations</name>
      <t><cref anchor="to-be-removed">RFC Editor: please replace RFCthis with the RFC
number of this RFC, and remove this note.</cref></t>
      <section anchor="cbor-tags">
        <name>CBOR tags</name> Tags</name>
        <t>In the "CBOR Tags" registry <xref target="IANA.cbor-tags"/>,
IANA has allocated the tags in <xref target="tab-tag-values"/> from what was at the
time the
FCFS space, with the present document as the specification reference.</t> target="tab-tag-values"/>.</t>
        <table anchor="tab-tag-values">
          <name>Values for Tags</name>
          <thead>
            <tr>
              <th align="right">Tag</th>
              <th align="left">Data Item</th>
              <th align="left">Semantics</th>
	      <th align="left">Reference</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td align="right">1001</td>
              <td align="left">map</td>
              <td align="left">[RFCthis] extended align="left">extended time</td>
	      <td align="left">[RFC9581, <xref target="time-format"/>]</td>
            </tr>
            <tr>
              <td align="right">1002</td>
              <td align="left">map</td>
              <td align="left">[RFCthis] duration</td> align="left">duration</td>
	      <td align="left">[RFC9581, <xref target="duration"/>]</td>
            </tr>
            <tr>
              <td align="right">1003</td>
              <td align="left">array</td>
              <td align="left">[RFCthis] period</td> align="left">period</td>
	      <td align="left">[RFC9581, <xref target="period"/>]</td>
            </tr>
          </tbody>
        </table>
        <t>IANA is requested to change has updated the "Data Item" column for Tag tag 1003 from
"map" to "array".</t>
      </section>
      <section anchor="timescale-registry">
        <name>Timescale
        <name>Timescales Registry</name>
        <t>This specification defines
        <t>Per this specification, IANA has created a new registry titled "Timescales" in registry within the
"CBOR
"Concise Binary Object Representation (CBOR) Tags" registry group <xref target="IANA.cbor-tags"/>, with a Registration Procedure
requiring target="IANA.cbor-tags"/>.  The registration procedure requires both
"Expert Review" and "RFC Required" (Sections <xref section="4.5" sectionFormat="bare" target="BCP26"/> target="RFC8126"/> and
<xref section="4.7" sectionFormat="bare" target="BCP26"/> target="RFC8126"/> of RFC 8126 <xref target="BCP26"/>).</t> target="BCP26"/>, respectively).</t>
        <t>Each entry needs to provide a timescale name (a sequence of uppercase
ASCII characters and digits, where a digit may not occur at the start:
<tt>[A-Z][A-Z0-9]*</tt>), a value (CBOR unsigned integer, uint,
0..18446744073709551615), a brief description
of the semantics, and a specification reference (RFC).
The initial contents are shown in <xref target="tab-timescales"/>.</t>
        <table anchor="tab-timescales">
          <name>Initial Content of Timescale Registry</name>
          <thead>
            <tr>
              <th align="left">Timescale</th>
              <th align="right">Value</th>
              <th align="left">Semantics</th>
              <th align="left">Reference</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td align="left">UTC</td>
              <td align="right">0</td>
              <td align="left">UTC with POSIX Epoch</td>
              <td align="left">[RFCthis]</td> align="left">[RFC9581]</td>
            </tr>
            <tr>
              <td align="left">TAI</td>
              <td align="right">1</td>
              <td align="left">TAI with PTP Epoch</td>
              <td align="left">[RFCthis]</td> align="left">[RFC9581]</td>
            </tr>
          </tbody>
        </table>
      </section>
      <section anchor="map-key-registry">
        <name>Time Tag Map Key Keys Registry</name>
        <t>This specification defines
        <t>Per this specification, IANA has created a new registry titled "Time Tag Map Keys"
in registry within the "CBOR "Concise Binary Object Representation (CBOR) Tags" registry group <xref target="IANA.cbor-tags"/>, with target="IANA.cbor-tags"/>.  The registration procedure  is "Specification
Required" as the Registration Procedure (<xref (Section <xref section="4.6" sectionFormat="of" sectionFormat="bare" target="RFC8126"/> of RFC 8126 <xref target="BCP26"/>).</t>
        <t>The designated expert is requested to assign the key values with the
shortest encodings (1+0 and 1+1 encoding) to registrations that are
likely to enjoy wide use and can benefit from short encodings.</t>
        <t>Each entry needs to provide a map key value (CBOR integer, int,
-18446744073709551616..18446744073709551615), a brief description
of the semantics, and a specification reference.
Note that negative integers indicate an elective key, while unsigned
integers indicate a key that either provides a base time or is
critical. The designated expert is requested to discuss with the registrant
 whether or not it is desirable to register a pair of an elective and a
 critical key for the same information, where the elective key value
 is the negative of the critical key (similar to how for example -11
 and 11 have been assigned in <xref target="tab-mapkeys"/>).
For the unsigned integers as keys, the choice of base time or critical
needs to be indicated in the brief semantics description.
(Elective map keys may be explicitly marked as such in the
description, e.g., to distinguish them from critical keys.)</t>
        <t>The initial contents are shown in <xref target="tab-mapkeys"/>.</t>
        <table anchor="tab-mapkeys">
          <name>Initial Content of Time Tag Map Keys Registry</name>
          <thead>
            <tr>
              <th align="right">Value</th>
              <th align="left">Semantics</th>
              <th align="left">Reference</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td align="right">-18</td>
              <td align="left">attoseconds</td>
              <td align="left">[RFCthis]</td> align="left">[RFC9581]</td>
            </tr>
            <tr>
              <td align="right">-15</td>
              <td align="left">femtoseconds</td>
              <td align="left">[RFCthis]</td> align="left">[RFC9581]</td>
            </tr>
	    <tr>
	      <td align="right">-13</td>
	      <td align="left">timescale (elective)</td>
	      <td align="left">[RFC9581]</td>
	    </tr>
            <tr>
              <td align="right">-12</td>
              <td align="left">picoseconds</td>
              <td align="left">[RFCthis]</td> align="left">[RFC9581]</td>
            </tr>
            <tr>
              <td align="right">-11</td>
              <td align="left">IXDTF Suffix Information (elective)</td>
              <td align="left">[RFCthis], align="left">[RFC9581], <xref target="IXDTF"/></td> target="RFC9557"/></td>
            </tr>
            <tr>
              <td align="right">-10</td>
              <td align="left">IXDTF Time Zone Hint (elective)</td>
              <td align="left">[RFCthis], align="left">[RFC9581], <xref target="IXDTF"/></td> target="RFC9557"/></td>
            </tr>
            <tr>
              <td align="right">-9</td>
              <td align="left">nanoseconds</td>
              <td align="left">[RFCthis]</td> align="left">[RFC9581]</td>
            </tr>
            <tr>
              <td align="right">-8</td>
              <td align="left">Guarantee</td>
              <td align="left">[RFCthis]</td> align="left">[RFC9581]</td>
            </tr>
            <tr>
              <td align="right">-7</td>
              <td align="left">Uncertainty</td>
              <td align="left">[RFCthis]</td> align="left">[RFC9581]</td>
            </tr>
            <tr>
              <td align="right">-6</td>
              <td align="left">microseconds</td>
              <td align="left">[RFCthis]</td> align="left">[RFC9581]</td>
            </tr>
            <tr>
              <td align="right">-5</td>
              <td align="left">Offset-Scaled Log Variance</td>
              <td align="left">[RFCthis]</td> align="left">[RFC9581]</td>
            </tr>
            <tr>
              <td align="right">-4</td>
              <td align="left">Clock Accuracy</td>
              <td align="left">[RFCthis]</td> align="left">[RFC9581]</td>
            </tr>
            <tr>
              <td align="right">-3</td>
              <td align="left">milliseconds</td>
              <td align="left">[RFCthis]</td> align="left">[RFC9581]</td>
            </tr>
            <tr>
              <td align="right">-2</td>
              <td align="left">Clock Class</td>
              <td align="left">[RFCthis]</td> align="left">[RFC9581]</td>
            </tr>
	    <tr>
	      <td align="right">-1</td>
	      <td align="left">timescale (elective) legacy</td>
	      <td align="left">[RFC9581]</td>
	    </tr>
            <tr>
              <td align="right">1</td>
              <td align="left">base time value as in CBOR Tag tag 1</td>
              <td align="left">
                <xref target="RFC8949"/> [RFCthis]</td> target="RFC8949"/>, [RFC9581]</td>
            </tr>
            <tr>
              <td align="right">4</td>
              <td align="left">base time value as in CBOR Tag tag 4</td>
              <td align="left">
                <xref target="RFC8949"/> [RFCthis]</td> target="RFC8949"/>, [RFC9581]</td>
            </tr>
            <tr>
              <td align="right">5</td>
              <td align="left">base time value as in CBOR Tag tag 5</td>
              <td align="left">
                <xref target="RFC8949"/> [RFCthis]</td> target="RFC8949"/>, [RFC9581]</td>
            </tr>
            <tr>
              <td align="right">10</td>
              <td align="left">IXDTF Time Zone Hint (critical)</td>
              <td align="left">[RFCthis], <xref target="IXDTF"/></td> align="left"><xref target="RFC9557"/>, [RFC9581]</td>
            </tr>
            <tr>
              <td align="right">11</td>
              <td align="left">IXDTF Suffix Information (critical)</td>
              <td align="left">[RFCthis], <xref target="IXDTF"/></td> align="left"><xref target="RFC9557"/>, [RFC9581]</td>
            </tr>
	    <tr>
	      <td align="right">13</td>
	      <td align="left">timescale (critical)</td>
	      <td align="left">[RFC9581]</td>
	    </tr>
          </tbody>
        </table>
      </section>
    </section>
    <section anchor="security-considerations">
      <name>Security Considerations</name>
      <t>The security considerations of <xref target="RFC8949"/> apply; the tags introduced
here are not expected to raise security considerations beyond those.</t>
      <t>Time, of course, has significant security considerations; these
include the exploitation of ambiguities where time is security
relevant (e.g., for freshness or in a validity span) or the disclosure
of characteristics of the emitting system (e.g., time zone, zone or clock
resolution and wall clock offset).</t>
      <t>A more detailed discussion of security considerations emanating from
using a representation of time that allows the inclusion of complex, complex and
possibly inconsistent information is available in
Section <xref "Security Considerations"
(<xref section="7" sectionFormat="bare" target="IXDTF">Security Considerations</xref> of <xref target="IXDTF"/>.</t> sectionFormat="of" target="RFC9557"></xref>).</t>
    </section>
  </middle>
  <back>
    <displayreference target="ISO8601" to="ISO8601:1988"/>
    <displayreference target="ISO8601-2019" to="ISO8601-1:2019"/>

    <references>
      <name>References</name>
      <references anchor="sec-normative-references">
        <name>Normative References</name>
        <reference anchor="RFC8949">
          <front>
            <title>Concise Binary Object Representation (CBOR)</title>
            <author fullname="C. Bormann" initials="C." surname="Bormann"/>
            <author fullname="P. Hoffman" initials="P." surname="Hoffman"/>
            <date month="December" year="2020"/>
            <abstract>
              <t>The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack.</t>
              <t>This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format.</t>
            </abstract>
          </front>
          <seriesInfo name="STD" value="94"/>
          <seriesInfo name="RFC" value="8949"/>
          <seriesInfo name="DOI" value="10.17487/RFC8949"/>
        </reference>
        <reference anchor="BCP26">
          <front>
            <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
            <author fullname="M. Cotton" initials="M." surname="Cotton"/>
            <author fullname="B. Leiba" initials="B." surname="Leiba"/>
            <author fullname="T. Narten" initials="T." surname="Narten"/>
            <date month="June" year="2017"/>
            <abstract>
              <t>Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA).</t>
              <t>To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry.</t>
              <t>This is the third edition of this document; it obsoletes RFC 5226.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="26"/>
          <seriesInfo name="RFC" value="8126"/>
          <seriesInfo name="DOI" value="10.17487/RFC8126"/>
        </reference>

	<xi:include
	    href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8949.xml"/>

	<referencegroup anchor="BCP26" target="https://www.rfc-editor.org/info/bcp26">
	<xi:include
	    href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8126.xml"/>
	</referencegroup>

        <reference anchor="TIME_T" target="http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_16">
          <front>
            <title>Vol. 1:
            <title>The Open Group Base Definitions, Specifications Issue 7</title>
            <author>
              <organization>The Open Group Base Specifications</organization>
              <organization>IEEE</organization>
            </author>
            <date year="2018"/>
          </front>
          <seriesInfo name="Section 4.16" value="'Seconds Since the Epoch'"/>
          <seriesInfo name="IEEE Std" value="1003.1-2017"/>
          <seriesInfo name="2018" value="Edition"/>
	  <refcontent>Section 4.16 Seconds Since the Epoch</refcontent>
        </reference>

        <reference anchor="SI-SECOND"> anchor="SI-SECOND" target="https://www.iso.org/standard/64974.html">
          <front>
            <title>Quantities and units  -- Part 3: Space and time</title>
            <author>
              <organization>International Organization for Standardization (ISO)</organization>
              <organization>ISO</organization>
            </author>
            <date day="01" month="March" year="2006"/>
          </front>
          <seriesInfo name="ISO" value="80000-3"/>
        </reference>
        <reference anchor="RFC8575">
          <front>
            <title>YANG Data Model for the Precision Time Protocol (PTP)</title>
            <author fullname="Y. Jiang" initials="Y." role="editor" surname="Jiang"/>
            <author fullname="X. Liu" initials="X." surname="Liu"/>
            <author fullname="J. Xu" initials="J." surname="Xu"/>
            <author fullname="R. Cummings" initials="R." role="editor" surname="Cummings"/>
            <date month="May" month="October" year="2019"/>
            <abstract>
              <t>This document defines a YANG data model for the configuration of devices and clocks using the Precision Time Protocol (PTP) as specified in IEEE Std 1588-2008. It also defines the retrieval of the configuration information, the data sets and the running states of PTP clocks. The YANG module in this document conforms to the Network Management Datastore Architecture (NMDA).</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8575"/>
          <seriesInfo name="DOI" value="10.17487/RFC8575"/> name="ISO" value="80000-3:2019"/>
        </reference>

<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8575.xml"/>

        <reference anchor="IEEE1588-2008" target="https://standards.ieee.org/ieee/1588/4355/">
          <front>
            <title>IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems</title>
            <author>
              <organization>IEEE</organization>
            </author>
            <date month="July" year="2008"/>
          </front>
          <seriesInfo name="IEEE" value="1588-2008"/>
          <annotation>Often called PTP v2, as it replaced the earlier 2002 version of this standard by a non-backwards compatible protocol.</annotation>
        </reference>

        <reference anchor="IEEE1588-2019" target="https://standards.ieee.org/ieee/1588/6825/">
          <front>
            <title>IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems</title>
            <author>
              <organization>IEEE</organization>
            </author>
            <date month="June" year="2020"/>
          </front>
          <seriesInfo name="IEEE" value="1588-2019"/>
          <annotation>Often called PTP v2.1, as it has been designed so it can be used in a way that is fully backwards compatible to IEEE1588-2008.</annotation>
        </reference>

        <reference anchor="GUM" target="https://www.bipm.org/en/publications/guides/gum.html">
          <front>
            <title>Evaluation of measurement data  -- Guide to the expression of uncertainty in measurement</title>
            <author>
              <organization>Joint Committee for Guides in Metrology</organization>
            </author>
            <date month="September" year="2008"/>
          </front>
          <seriesInfo name="JCGM" value="100:2008"/>
        </reference>

        <reference anchor="IANA.cbor-tags" target="https://www.iana.org/assignments/cbor-tags">
          <front>
            <title>Concise Binary Object Representation (CBOR) Tags</title>
            <author>
              <organization>IANA</organization>
            </author>
          </front>
        </reference>
        <reference anchor="RFC8610">
          <front>
            <title>Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures</title>
            <author fullname="H. Birkholz" initials="H." surname="Birkholz"/>
            <author fullname="C. Vigano" initials="C." surname="Vigano"/>
            <author fullname="C. Bormann" initials="C." surname="Bormann"/>
            <date month="June" year="2019"/>
            <abstract>
              <t>This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8610"/>
          <seriesInfo name="DOI" value="10.17487/RFC8610"/>
        </reference>
        <reference anchor="IXDTF">
          <front>
            <title>Date and Time on the Internet: Timestamps with additional information</title>
            <author fullname="Ujjwal Sharma" initials="U." surname="Sharma">
              <organization>Igalia, S.L.</organization>
            </author>
            <author fullname="Carsten Bormann" initials="C." surname="Bormann">
              <organization>Universität Bremen TZI</organization>
            </author>
            <date day="23" month="October" year="2023"/>
            <abstract>
              <t>   This document defines an extension to the timestamp format defined in
   RFC3339 for representing additional information including a time
   zone.

   It updates RFC3339 in the specific interpretation of the local offset
   Z, which is no longer understood to "imply that UTC is the preferred
   reference point for the specified time"; see Section 2.

   // (This "cref" paragraph will be removed by the RFC editor:) The
   // present version (-11) addresses comments received during IESG
   // review.

              </t>
            </abstract>
          </front>
          <seriesInfo name="Internet-Draft" value="draft-ietf-sedate-datetime-extended-11"/>
        </reference>
        <reference anchor="RFC2119">
          <front>
            <title>Key words for use in RFCs to Indicate Requirement Levels</title>
            <author fullname="S. Bradner" initials="S." surname="Bradner"/>
            <date month="March" year="1997"/>
            <abstract>
              <t>In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="14"/>
          <seriesInfo name="RFC" value="2119"/>
          <seriesInfo name="DOI" value="10.17487/RFC2119"/>
        </reference>
        <reference anchor="RFC8174">
          <front>
            <title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</title>
            <author fullname="B. Leiba" initials="B." surname="Leiba"/>
            <date month="May" year="2017"/>
            <abstract>
              <t>RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="14"/>
          <seriesInfo name="RFC" value="8174"/>
          <seriesInfo name="DOI" value="10.17487/RFC8174"/>
        </reference>

<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8610.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9557.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>

      </references>

<references anchor="sec-informative-references">
        <name>Informative References</name>
        <reference anchor="RFC3161">
          <front>
            <title>Internet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP)</title>
            <author fullname="C. Adams" initials="C." surname="Adams"/>
            <author fullname="P. Cain" initials="P." surname="Cain"/>
            <author fullname="D. Pinkas" initials="D." surname="Pinkas"/>
            <author fullname="R. Zuccherato" initials="R." surname="Zuccherato"/>
            <date month="August" year="2001"/>
            <abstract>
              <t>This document describes the format of a request sent to a Time Stamping Authority (TSA) and of the response that is returned. It also establishes several security-relevant requirements for TSA operation, with regards to processing requests to generate responses. [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3161"/>
          <seriesInfo name="DOI" value="10.17487/RFC3161"/>
        </reference>
        <reference anchor="RFC3339">
          <front>
            <title>Date and Time on the Internet: Timestamps</title>
            <author fullname="G. Klyne" initials="G." surname="Klyne"/>
            <author fullname="C. Newman" initials="C." surname="Newman"/>
            <date month="July" year="2002"/>
            <abstract>
              <t>This document defines a date and time format for use in Internet protocols that is a profile of the ISO 8601 standard for representation of dates and times using the Gregorian calendar.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3339"/>
          <seriesInfo name="DOI" value="10.17487/RFC3339"/>
        </reference>

<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.3161.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.3339.xml"/>

        <reference anchor="ISO8601" target="https://www.iso.org/standard/15903.html">
          <front>
            <title>Data elements and interchange formats  -- Information interchange  -- Representation of dates and times</title>
            <author>
              <organization abbrev="ISO">International Organization for Standardization</organization>
              <organization>ISO</organization>
            </author>
            <date month="June" year="1988"/>
          </front>
          <seriesInfo name="ISO" value="8601:1988"/>
          <annotation>Also available from &lt;⁠<eref &lt;<eref target="https://nvlpubs.nist.gov/nistpubs/Legacy/FIPS/fipspub4-1-1991.pdf">https://nvlpubs.nist.gov/nistpubs/Legacy/FIPS/fipspub4-1-1991.pdf</eref>&gt;.</annotation>
        </reference>

        <reference anchor="ISO8601-2019" target="https://www.iso.org/standard/70907.html">
          <front>
            <title>Date and time  -- Representations for information interchange  -- Part 1: Basic rules</title>
            <author>
              <organization abbrev="ISO">International Organization for Standardization</organization>
              <organization>ISO</organization>
            </author>
            <date month="February" year="2019"/>
          </front>
          <seriesInfo name="ISO" value="8601-1:2019"/>
        </reference>

        <reference anchor="C" target="https://www.iso.org/standard/74528.html">
          <front>
            <title>Information technology  -- Programming languages  -- C</title>
            <author>
              <organization>International Organization for Standardization</organization>
              <organization>ISO</organization>
            </author>
            <date month="June" year="2018"/>
          </front>
          <seriesInfo name="ISO/IEC" value="9899:2018"/>
          <annotation>Contents available via &lt;⁠<eref target="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf">https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf</eref>&gt;</annotation> &lt;<eref target="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf"/>&gt;</annotation>
          <refcontent>Fourth Edition</refcontent>
        </reference>

<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5905.xml"/>

<reference anchor="RFC5905"> anchor="IEEE754" target="https://ieeexplore.ieee.org/document/8766229">
  <front>
            <title>Network Time Protocol Version 4: Protocol and Algorithms Specification</title>
            <author fullname="D. Mills" initials="D." surname="Mills"/>
            <author fullname="J. Martin" initials="J." role="editor" surname="Martin"/>
            <author fullname="J. Burbank" initials="J." surname="Burbank"/>
            <author fullname="W. Kasch" initials="W." surname="Kasch"/>
    <title>IEEE Standard for Floating-Point Arithmetic</title>
    <author>
       <organization>IEEE</organization>
    </author>
    <date month="June" year="2010"/>
            <abstract>
              <t>The Network Time Protocol (NTP) is widely used to synchronize computer clocks in the Internet. This document describes NTP version 4 (NTPv4), which is backwards compatible with NTP version 3 (NTPv3), described in RFC 1305, as well as previous versions of the protocol. NTPv4 includes a modified protocol header to accommodate the Internet Protocol version 6 address family. NTPv4 includes fundamental improvements in the mitigation and discipline algorithms that extend the potential accuracy to the tens of microseconds with modern workstations and fast LANs. It includes a dynamic server discovery scheme, so that in many cases, specific server configuration is not required. It corrects certain errors in the NTPv3 design and implementation and includes an optional extension mechanism. [STANDARDS-TRACK]</t>
            </abstract> month="July" year="2019"/>
  </front>
  <seriesInfo name="RFC" value="5905"/> name="IEEE" value="754-2019"/>
  <seriesInfo name="DOI" value="10.17487/RFC5905"/> value="10.1109/IEEESTD.2019.8766229"/>
</reference>

     </references>
    </references>
    <?line 895?>

<section anchor="collected-cddl">
      <name>Collected CDDL</name>
      <t>This appendix collects the CDDL rules spread over the document into
one convenient place.</t>
      <figure anchor="fig-collected-cddl">
        <name>Collected CDDL rules Rules from this specification</name> This Specification</name>
        <sourcecode name="time-tag-collected-cddl.cddl" type="cddl"><![CDATA[
Etime = #6.1001(etime-detailed)

etime-framework = {
  uint => any ; at least one base time
  * (nint/text) => any ; elective supplementary information
  * uint => any ; critical supplementary information
}

etime-detailed = ({
  $$ETIME-BASETIME
  ClockQuality-group
  * $$ETIME-ELECTIVE
  * $$ETIME-CRITICAL
  * ((nint/text) .feature "etime-elective-extension") => any
  * (uint .feature "etime-critical-extension") => any
}) .within etime-framework

$$ETIME-BASETIME //= (1: ~time)

$$ETIME-BASETIME //= (4: ~decfrac)
$$ETIME-BASETIME //= (5: ~bigfloat)

$$ETIME-ELECTIVE //= (-3: uint)
$$ETIME-ELECTIVE //= (-6: uint)
$$ETIME-ELECTIVE //= (-9: uint)
$$ETIME-ELECTIVE //= (-12: uint)
$$ETIME-ELECTIVE //= (-15: uint)
$$ETIME-ELECTIVE //= (-18: uint)

$$ETIME-ELECTIVE //= (-1 => $ETIME-TIMESCALE)
$$ETIME-ELECTIVE //= (-13 => $ETIME-TIMESCALE)
$$ETIME-CRITICAL //= (13 => $ETIME-TIMESCALE)

$ETIME-TIMESCALE /= &(etime-utc: 0)
$ETIME-TIMESCALE /= &(etime-tai: 1)

ClockQuality-group = (
  ? &(ClockClass: -2) => uint .size 1 ; PTP/RFC8575
  ? &(ClockAccuracy: -4) => uint .size 1 ; PTP/RFC8575
  ? &(OffsetScaledLogVariance: -5) => uint .size 2 ; PTP/RFC8575
  ? &(Uncertainty: -7) => ~time/~duration
  ? &(Guarantee: -8) => ~time/~duration
)

$$ETIME-ELECTIVE //= (-10: time-zone-info)
$$ETIME-CRITICAL //= (10: time-zone-info)

time-zone-info = tstr .abnf
                 ("time-zone-name / time-numoffset" .det IXDTFtz)
IXDTFtz = '
   time-hour       = 2DIGIT  ; 00-23
   time-minute     = 2DIGIT  ; 00-59
   time-numoffset  = ("+" / "-") time-hour ":" time-minute

   time-zone-initial = ALPHA / "." / "_"
   time-zone-char    = time-zone-initial / DIGIT / "-" / "+"
   time-zone-part    = time-zone-initial *13(time-zone-char) *time-zone-char
                       ; but not "." or ".."
   time-zone-name    = time-zone-part *("/" time-zone-part)
   ALPHA             =  %x41-5A / %x61-7A   ; A-Z / a-z
   DIGIT             =  %x30-39 ; 0-9
' ; extracted from [IXDTF] [RFC9557] and [RFC3339]; update as needed [RFC3339]

$$ETIME-ELECTIVE //= (-11: suffix-info-map)
$$ETIME-CRITICAL //= (11: suffix-info-map)

suffix-info-map = { * suffix-key => suffix-values }
suffix-key = tstr .abnf ("suffix-key" .det IXDTF)
suffix-values = one-or-more<suffix-value>
one-or-more<T> = T / [ 2* T ]
suffix-value = tstr .abnf ("suffix-value" .det IXDTF)

IXDTF = '
   key-initial       = lcalpha / "_"
   key-char          = key-initial / DIGIT / "-"
   suffix-key        = key-initial *key-char

   suffix-value      = 1*alphanum
   alphanum          = ALPHA / DIGIT
   lcalpha           =  %x61-7A
   ALPHA             =  %x41-5A / %x61-7A   ; A-Z / a-z
   DIGIT             =  %x30-39 ; 0-9
' ; extracted from [IXDTF]; update as needed! [RFC9557]

Duration = #6.1002(etime-detailed)

Period

simple-Period = #6.1003([
  start: ~Etime / null
  end: ~Etime / null
  ? duration: ~Duration / null
])

clumsy-Period

Period = #6.1003([
  (start: ~Etime,
   ((end: ~Etime,
     ? duration: null) ~Etime) //
    (end: null,
     duration: ~Duration))) //
  (start: null,
   end: ~Etime,
   duration: ~Duration)
])

etime = #6.1001({* (int/tstr) => any})
duration = #6.1002({* (int/tstr) => any})
period = #6.1003([~etime/null, ~etime/null, ~duration/null]) ?~duration])
]]></sourcecode>
      </figure>
    </section>
    <section anchor="acknowledgements" numbered="false">
      <name>Acknowledgements</name>
      <t>The authors would like to acknowledge the many comments from members
of the CBOR WG,
<contact fullname="Francesca Palombini"/> for her AD review, and
<contact fullname="Thomas Fossati"/> and
<contact fullname="Qin Wu"/> for their directorate reviews.</t>
      <!--  LocalWords:  CBOR extensibility IANA uint sint IEEE endian TAI
 -->
<!--  LocalWords:  signedness endianness NTP IXDTF
 --> reviews, and <contact fullname="Rohan Mahy"/> for one more review late in the process.</t>
    </section>
  </back>
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