What this calculator does
This tool tells you which calendar date falls a chosen number of days before a base date, and which day of the week that is. Enter a base date and a count N, and it subtracts the days — correctly crossing month and year boundaries and respecting leap years — using serial-day (Julian Day Number) arithmetic. The date math itself is universal and works for any reader worldwide.
Japanese era (wareki) note
The optional Calendar / Era selector is a Japan-specific convenience. Choose Western (CE) (the default) to enter the Gregorian year directly. Or pick a Japanese imperial era — Meiji, Taisho, Showa, Heisei, or Reiwa — and type the era year; the tool adds the era offset to get the Gregorian year (for example Reiwa 6 = 2024). Era input is only meaningful from Meiji 6 (1873), when Japan adopted the Gregorian calendar.
How to use it
1) Pick the calendar/era. 2) Enter the year, month, and day of your base date. 3) Enter how many days before (N). 4) Choose whether the base day itself counts as day 1. The "Count" option is useful for inclusive counting such as Japanese mourning anniversaries (for example the 49th-day, "shijukunichi"). Tip: entering a negative N computes a future date instead.
The formula
GregorianYear = enteredYear + eraOffset. The base date is converted to a serial day number; delta = N when the first day is not counted, or N−1 when it is counted; resultDate = baseDate − delta days. The weekday comes from the serial number modulo 7.
Worked example
Base date 2024-06-15, N = 10, first day not counted. delta = 10, so
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