Date Calculator: Add or Subtract Time From Any Date

Adding time to a date is not like adding ordinary numbers, because months have different lengths and years have leap days. Adding days is straightforward: count forward day by day, crossing month and year boundaries as needed. Adding months is trickier: January 31 plus 1 month has no February 31 to land on, so the result is clamped to the last day of February, which is February 28 in a common year and February 29 in a leap year.

This calculator adds years and months first, with clamping to the target month's last day, then adds or subtracts days. That order matches how most date calculators and calendar apps handle month arithmetic, and it avoids the surprise of January 31 plus 1 month landing in March. All calculations use UTC dates, so time zones and daylight saving changes never shift the result by a day.

For the gap between two known dates, a date difference calculator counts days, weeks, and months between them.

To add years, keep the month and day the same and increase the year, except for February 29, which becomes February 28 in a non-leap year. To add months, increase the month count, carrying into the year every 12 months, and clamp the day to the target month's length. To add days, move forward one day at a time in the calendar, which automatically handles month lengths and leap years.

Subtraction works the same way in reverse: subtract years and months with clamping, then subtract days. A useful check is to run the calculation backward: the result plus the subtracted amount, or minus the added amount, should return the start date, except when clamping changed the day, as with January 31 plus 1 month.

Example 1: October 27, 2026 plus 90 days. Counting forward through November and December lands on January 25, 2027, a Monday. Example 2: January 31, 2026 plus 1 month. February 2026 has 28 days, since 2026 is not a leap year, so the result is February 28, 2026, a Saturday.

Example 3: March 15, 2026 plus 1 year, 2 months, and 15 days. Adding years and months first gives May 15, 2027, then adding 15 days gives May 30, 2027. Example 4: January 10, 2026 minus 10 days. Counting backward gives December 31, 2025, crossing the year boundary.

A year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. So 2024 is a leap year, 2026 is not, 2000 was a leap year, and 2100 will not be. Leap years add February 29, which is why February has 29 days in 2024 and 2028, and 28 days in other years.

Month lengths are fixed except February: January, March, May, July, August, October, and December have 31 days, while April, June, September, and November have 30 days. The old knuckle mnemonic, 31 day months on knuckles and 30 day months in the gaps, still works for remembering them without a calendar.

The most common mistake is treating every month as 30 days: 3 months is not always 90 days, since three calendar months can span 89, 90, 91, or 92 days depending on which months they are. Another mistake is forgetting leap years, which shifts dates after February in a leap year by one day compared with a common year.

A third mistake is expecting January 31 plus 1 month to land on a specific March date. Different tools handle the overflow differently, some clamp to February 28 and some roll into March, so check which rule your tool uses before relying on a month-end result. This calculator clamps, and it says so in the result note.

Date calculators plan deadlines, subscriptions, warranties, and project schedules: a 90 day trial starting October 27, 2026 ends January 25, 2027, and a 6 month contract starting March 15 ends September 15. They also plan travel, visa stays, and billing cycles, where the exact end date matters more than the rough month count.

For the weekday of any single date, a day of week calculator answers without any addition. Use this date calculator whenever a start date plus a duration, or a start date minus a duration, is the question.