Japanese to Western Calendar Conversion (One Month)

・The old calendar is based on historical records and aligns with the historical lunisolar calendar.
・For era name changes up to the Meiji era, the first year of the new era is retroactively set to New Year’s Day of the year in which the change was proclaimed. (New era name effective from the start of the year)
*In years marking a transition between era names, both are listed side by side. (Example: Genna 10 / Kan’ei 1)
・On Meiji 5 year 12 month 3 day, the date corresponds to Meiji 6 year January 1 in the Gregorian calendar; from that point onward, Japanese calendar dates correspond to the current Gregorian calendar.
・For periods without an era name system, the number of years since the Emperor’s accession is used instead. (Suiko, Jōmei, Kōgyoku, Saimei, Tenji, Tenmu, Jitō, Monmu)
・For the Northern and Southern Courts period (1331–1392), the era names of both the Northern Court and the Southern Court are listed side by side.
・For Western dates, the Julian calendar is used until 1582-10-04; the Gregorian calendar is adopted from the following day onward.
　(The date 1582 year 10 month 5 day in the Julian calendar corresponds to 1582 year 10 month 15 day in the Gregorian calendar.)

Reference: “Encyclopedia of Calendars,” Tokyodo Publishing