Date Calculator

1. Enter the starting date.
2. Choose the unit — days, weeks, months or years — and how much to add.
3. The result shows the final date and its day of the week.

Adding one month moves to the same day of the following month, whether it has 28, 30 or 31 days. That is why 31 January plus one month does not land on 2 March.

When the target day does not exist, the date is clamped to the last available day: 31 January plus one month gives 28 February, or 29 in a leap year.

That clamping makes the operation non-reversible. Adding a month and then subtracting one may not return the original date, precisely because the day was adjusted along the way.

If your deadline counts in calendar days, use the days unit — the result will differ and that is the correct one for that case.

Adding one year to 29 February lands on 28 February of the following year, because the target date does not exist outside leap years.

The leap year rule is not simply "every four years". Years divisible by 100 are not leap years, except those divisible by 400. That is why 1900 was not a leap year and 2000 was.

The calculation here uses the browser’s own calendar, which implements that rule correctly.

1. Enter the reference date.
2. Choose the unit — days, weeks, months or years — and how much to subtract.
3. The result shows the earlier date and its day of the week.

Subtracting dates is the operation for anyone starting from a due date and needing to know when to begin. It is the backwards calculation used in project schedules, notice periods and scheduling stages ahead of a delivery.

The typical question is: if delivery is on the 30th and production takes 45 days, when do I start? Subtracting 45 days from the final date answers it directly.

Note the result is in calendar days. If your term counts business days, the real start date will be considerably earlier — use the business day calculator for that count.

Subtracting one month steps back to the same day of the previous month. When that day does not exist, the date is clamped to the last available one.

31 March minus one month gives 28 or 29 February, not 3 March. The clamping is the same as in addition, and for the same reason.

The practical consequence is that subtracting a month and then adding one may not return the starting date. If your calculation must be exactly reversible, work in days.

Subtracting years from today is how you find the birth date matching an age — useful for checking whether someone meets a minimum age requirement.

Watch out for 29 February: subtracting a year from that date lands on 28 February, because the day does not exist in common years.

All calculation happens in your browser; no date is transmitted.