How this calculator works
This calculator moves a date forward or backward by a number of days, weeks, months or years — useful for working out a deadline, a return date, an anniversary, or any date that is defined as "N units from" another date. The result is shown with its full weekday name, so you know immediately whether it lands on a weekend.
Days and weeks are exact: adding 14 days always lands exactly two weeks later, with no ambiguity. Months and years are trickier, because calendar months have different lengths — adding a month to the 31st of a month can't land on a 31st in a shorter month, so the day-of-month is clamped to the last valid day instead of overflowing into the next month.
How each unit is applied
Days: new date = start date ± N days (exact, since a day is a fixed unit).Weeks: new date = start date ± (N × 7) days.Months and years: step the month (and year) counter by N (or N × 12 for years), then clamp the day-of-month to the last day of the resulting month if it doesn't have that many days.The clamping rule means Jan 31 + 1 month = Feb 28 in a common year but Feb 29 in a leap year — and it also means adding a month and then subtracting a month does not always return you to the exact original day.
Worked example: month-end clamping
- Start date: January 31, 2024. Operation: add 1 month.
- February 2024 has 29 days (2024 is a leap year: divisible by 4, and not a skipped century year), so the day-of-month clamps from 31 to 29.
- Result: February 29, 2024, a Thursday.
- For comparison, January 31, 2023 (not a leap year) plus 1 month clamps to February 28, 2023 instead.
Frequently asked questions
Why didn't adding 1 month to Jan 31 give me March 2 or 3?
Some spreadsheet functions add 31 days rather than 1 calendar month, which does overflow into March. This calculator adds a genuine calendar month and clamps the day, matching how most people mean "one month later."
Can I subtract a date instead of adding one?
Yes — the Add/Subtract control switches the direction. Subtracting works the same way, including the same month-end clamping in reverse (e.g. March 31 minus 1 month lands on February 28 or 29).
Does the calculator account for leap years automatically?
Yes. Whether the target February has 28 or 29 days is determined by the standard Gregorian leap-year rule (divisible by 4, except century years unless divisible by 400) every time, not hard-coded.
What if I need to know the difference between two existing dates instead?
Use the Date Difference Calculator instead — this one takes a single date and moves it, while that one compares two dates you already have.
What happens if I add exactly 1 year to a leap day like February 29, 2024?
It lands on February 28, 2025, since 2025 is not a leap year and has no February 29 to land on directly. The same day-of-month clamping rule this calculator uses for months also applies when adding whole years: if the resulting year's February doesn't have 29 days, the date clamps down to February 28 instead of overflowing into March.