moment.updateLocale('en', { eras: [{ since: '0001-01-01', until: +Infinity, offset: 1, name: 'Anno Domini', narrow: 'AD', abbr: 'AD' }, { until: -Infinity, since: '0000-12-31', offset: 1, name: 'Before Christ', narrow: 'BC', abbr: 'BC' }], });

Specify Eras for a particular locale. An era is a time interval with name and year numbering. Absolute year number (like 2020) can also be specified as 2020 AD: the 2020th year of the era AD. Similarly the absolute year number -0500 can be described as 501 BC, the 501st year from the BC era.

eras: [{
    since:  '0001-01-01', // the start of the era
    until:  +Infinity,    // the end of the era, can be +/-Infinity
    offset: 1,            // added to year to (mostly) avoid 0 era years
    name:   'Anno Domini',// full name of era
    narrow: 'AD',         // narrow name of era
    abbr:   'AD'          // abbreviated name of era

since and until govern the direction of the era. As in the case of BC it grows toward -Infinity, thus since > until. For eras that increment toward +Infinity since < until.

Parsing/formatting of eras is accomplished with yo, y* and N* tokens.

Note: The era-related APIs are subject to change.