keisoku

Add or Subtract Years & Months from a Date

Add or subtract years and months from any date and get the result with the weekday, Japanese era, and number of days elapsed. Handles month-end adjustment automatically.

Input

YYYY/MM/DD
yr
mo

Result

Enter a base date to begin.

How it works

  • Pick a base date, enter the number of years and months you want to add (or subtract), and switch between 'After (add)' and 'Before (subtract)' to see the resulting date with its weekday.
  • You can enter just years or just months, and setting both to 0 returns the base date itself.
  • End-of-month handling: if the target month does not contain the same day, the result is rounded to the last day of that month. For example, one month after January 31 becomes the last day of February (28 in a common year, 29 in a leap year), since February has no 31st.
  • Because years and months are applied first and the day is matched afterward, adding and subtracting may not be exact inverses (for instance when an end-of-month adjustment occurs).
  • The result also shows the number of days elapsed from the base date and the Japanese era (Reiwa, Heisei, and so on), which is handy for confirming maturity dates or anniversaries.
  • The Japanese era is determined by the date the era changed. From the day of the era change onward, the new era name is shown.