The moment a calendar counts from — and the reason year numbers disagree.
An epoch is the fixed point a calendar's year count begins at. Gregorian years count from a computed date of the Nativity; the Hijri calendars from the migration to Medina; the Ethiopian calendar from a different reckoning of the Annunciation, which is why its year number sits seven to eight years behind.
The epoch is separate from the arithmetic. Two calendars can share an epoch and run differently — the lunar and solar Hijri calendars do — and two can share arithmetic and differ only in where they start, as the Coptic and Ethiopian calendars nearly do.
In software the word has a second life: the Unix epoch, 1 January 1970 UTC, from which timestamps are counted. Same idea, different zero.
An epoch is the instant a count begins. An era is a named span counted from one — a Japanese reign era, for instance, whose numbering restarts.
See also gregorian calendar · hijri calendar · solar hijri calendar · ethiopian calendar
We hold this as implicit in each rule_type rather than stored. See what we hold.