Japanese Calendar Date (Wareki)

The page works in both directions. Feed it a Gregorian date from 1 January 1873 onward and you get the era and the era year, the full wareki date, the Japanese written form, the first day of that era and the weekday. Or pick an era from the list, type a year inside it, and read off the Gregorian year. Everything comes from the Intl japanese calendar, which carries the CLDR era table, instead of a fixed subtraction that only holds until the next accession.

One era per reign has been the rule since 1868, and the count restarts at 1 when a new emperor takes the throne. Taishō opens on 30 July 1912, Shōwa on 25 December 1926, Heisei on 8 January 1989 and Reiwa on 1 May 2019, the last one following an abdication, the first in Japan since 1817. Because the changes land mid-year, some era years are very short: Shōwa 64 lasted seven days, from 1 to 7 January 1989, and Reiwa 1 ran only from May to December.

This is where year-to-year tables break: 30 April 2019 is Heisei 31 while 1 May 2019 is Reiwa 1. The page asks for the whole date for exactly that reason. The first year of any era is written 元年, never 1年, and the output follows that convention. The range starts in 1873 because that is when Japan switched calendars: the day after Meiji 5, twelfth month, second day became 1 January of Meiji 6. Earlier dates sit on the lunisolar Tenpō calendar, and the page refuses them instead of guessing. Rokuyō day labels are also out of scope.