User Manual
Date Calculator User Manual
Use Date Calculator to count elapsed calendar days between two dates or to add and subtract Gregorian calendar units from a date.
Between dates
Choose Between dates, enter a Start date and End date, and read the primary Days between result. End must be on or after Start. If the dates are reversed, use Swap rather than expecting the app to silently reorder them.
The supporting results show Calendar difference, Weeks and days, Inclusive date count, and the weekday for each endpoint.
Add or subtract
Choose Add or subtract, enter a Base date, choose Add or Subtract, then enter whole non-negative Years, Months, Weeks, and Days. Blank duration fields mean zero. Years and months are combined into one calendar-month displacement and constrained to the last valid date in the destination month; weeks and days are applied afterward.
Month-end and leap-day behavior
2020-01-31 + 1 month becomes 2020-02-29. 2020-01-31 + 2 months becomes 2020-03-31. 2024-02-29 + 1 year becomes 2025-02-28.
Range and validation
Dates must be valid Gregorian dates from 0001-01-01 through 9999-12-31. Offset components accept whole numbers from 0 through 999,999,999, but any calculation whose result would leave the supported date range is rejected.
Copy, reset, and page memory
Copy result becomes available only for a valid calculation. If browser clipboard access is unavailable, the app exposes the same result for manual copying. Reset clears the active mode and asks before discarding meaningful work. Calculator work is page-memory only.