A year count that restarts with each emperor — so year 1 recurs.
Japan numbers years by imperial era. Reiwa began in 2019 and its year 1 is that year; the previous era, Heisei, ran from 1989. Era names and their start dates are set by law, and official Japanese documents are dated in them.
The property that matters for software is that the count RESTARTS. Year 1 is not unique, and a date given as an era year without the era name is ambiguous across decades.
Era changes are also, unusually, scheduled events that a system must be updated for. The transition to Reiwa required patching date libraries across the Japanese software estate, on a known date, which is a rare case of a calendar change with a project plan.
An epoch is fixed once. A Japanese era begins a new count with each reign, so the same era year number recurs.
See also epoch · buddhist era · gregorian calendar
We hold this as not stored — dates are published Gregorian. See what we hold.