era                     Create an era object
era_parameters          Get parameters of an era
era_year                Year units
era_year_parameters     Get the parameters of an 'era_year' object.
eras                    Standard era definitions
is_era                  Validation functions for 'era' objects
is_era_year             Validation functions for 'era_year' objects
is_yr                   Validation functions for 'yr' objects
this_year               Current year
yr                      Create a vector of years with era
yr_difference           Chronological difference between year vectors
yr_earlier_than         Chronological comparison of years
yr_era                  Get or set the era of a vector of years
yr_extremes             Chronological minima and maxima
yr_sort                 Chronological ordering of year vectors
yr_transform            Convert years from one era to another
