Invalid dates in Japanese eras?

Kip Cole kipcole9 at
Fri Sep 10 21:15:14 CDT 2021

For posterity, here is my current understanding after digging deeper into the CLDR data for Japanese eras:

*  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 that I could see)

For other calendars:

*   coptic, ethiopic, islamic, islamic_civil, islamic_rgsa, islamic_tbla, islamic_umalqura the era dates appear to be Julian dates (Julian day, month and year)

*   persian appears to be Julian year with persian month and day

*   Gregorian is, well, Gregorian date

When time permits I will submit a PR to CLDR

> On 9 Sep 2021, at 5:42 pm, Kip Cole <kipcole9 at> wrote:
> Ahhh, thank you so much - that makes sense to me. Here was I thinking about “y" from a formatting point of view. So basically “extended year” would mean “proleptic Gregorian year” for the given date in any calendar?
> Very helpful, and makes sense (and hopefully is even correct!)
> Regards, —Kip
>> On 9 Sep 2021, at 5:39 pm, Edward Welbourne <edward.welbourne at> wrote:
>> Wáng Yifán (09 September 2021 05:07) wrote:
>>> [...] I couldn't find the exact definition of "extended Gregorian year" elsewhere.
>> My semi-educated guess is that "extended Gregorian year" means the year
>> that the relevant date would fall in, in the "extended Gregorian
>> calendar" - i.e. the Gregorian calendar extrapolated backwards, if
>> necessary, to before Gregory introduced it.
>> 	Eddy.

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