View Source Cldr.Calendar.Era (Cldr Calendars v1.20.0)
Encapsulates the era information for known CLDR calendars.
The era data in CLDR is presented in different calendars at different times. The current best understanding is:
From Taisho (1912), the date is Gregorian year, month and day.
For Tenshō (Momoyama period) to Meji era (1868) inclusive its Gregorian year but lunar month and day.
For earlier eras it is Julian year with lunar month and day (but the Julian and Gregorian years coincide; there are no era dates where the Gregorian year would be different to the Julian year).
For Chinese and Korean (dangi) the era dates are Gregorian year with lunar month and lunar day
For Coptic, Ethiopic and Islamic calendars the eras are Julian dates (Julian day, month and year).
Persian era is Julian year with persian month and day.
Gregorian is, well, Gregorian date.