Start date for the Meiji era is wrong for the Japanese Calendar

🛬 Merged PR

@robertbastian merged a PR to unicode-org/cldr:main

CLDR-11375 Fix Meiji start date (#4610)

Accepted in TC 2025-05-07

This issue was reported to ICU4X.

Trac Comment 9 by mjansche@1d5920f4b44b27a8—2019-05-07T18:02:48.594Z

One more thing: If I take a Gregorian date such as 1831-1-23 and transpose that into any Japanese calendar (modern Gregorian or traditional lunisolar) and format that in terms of era plus year, I would expect to see 天保元年 as part of the output (regardless of whether I look at month and day, which would differ between modern and traditional calendar). In other words:

Input: 1831-1-23 Gregorian
Action: convert to Japanese calendar, then format as era plus year, drop date and month
Expected: 天保元年
Observed: 天保2年

Japanese Wikipedia explains this, but arguably English Wikipedia muddles the issue.

Trac Comment 8 by mjansche@1d5920f4b44b27a8—2019-05-07T17:39:18.503Z

I would argue it's simplest to change all listed dates before 1873 to Gregorian dates. Then the Gregorian calendar classes in ICU (including JapaneseCalendar) will align the era changes correctly to their Gregorian dates.

This will not necessarily correspond to historical practice, but that's okay, since JapaneseCalendar clearly states that it is fundamentally a Gregorian calendar. It is not the same as the traditional Japanese lunisolar calendar, which could be implemented separately if desired.

Aside: According to the National Diet Library, the change to Gregorian calendar took effect at the start of the Gregorian year 1873, which became the 6th year of Meiji. The traditional Japanese calendar would refer to Gregorian 1872-12-31 as 明治5年12月2日 and to 1873-1-1 as 明治6年1月1日 (instead of 明治5年12月3日). Support for lunisolar dates would have to take this change into account, which is arguably a separate issue.