Spans, business days, age and Japanese era conversion in one place. Japanese public holidays are computed under the law as it stood in that year — every amendment since 1949, the days the Olympics moved, and the one-off holidays. Everything runs inside your browser; no date you enter is sent anywhere.
Start date
End date
Base date
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When adding months or years lands on a date that does not exist — one month after 31 January, say — the result is clamped to the last day of that month, which is how the Japanese Civil Code counts periods.
Date of birth
As of (today by default)
School years in Japan group everyone born from 2 April to 1 April of the following year. The 1 April boundary comes from the legal rule that a person turns a year older on the day before their birthday.
Western (Gregorian) date
Japanese era → Western
year
month
day
Year
Supported range: 1949 to 2099
Computed under the law of that year. The Holidays Act has been amended many times, so applying today's rules to an older year gives the wrong dates.
Coming of Age Day was 15 January until 1999; the Emperor's Birthday was 23 December throughout the Heisei era; Marine Day did not exist before 1996;
Health and Sports Day was 10 October until 1999. In 2020 and 2021 the Olympics moved Marine Day, Mountain Day and Sports Day.
Substitute holidays start on 12 April 1973 and citizens' holidays in 1986. The six one-off holidays (enthronement ceremonies and the like) are included.
All of this also feeds the business-day count on the first tab.