Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Add or subtract days, weeks, months, or years from any date.
2026-01-31
This calculator projects a new date by adding or subtracting a chosen number of days, weeks, months, or years from a start date -- useful for planning deadlines, contract terms, warranty expirations, or any "what date is 90 days from now" question. It correctly handles month-length differences (adding a month to January 31 doesn't silently overflow into March) and leap years.
Pick a start date, choose Add or Subtract, enter an amount, and select a unit (days, weeks, months, or years). The result updates instantly with the resulting date and its day of the week.
Days and weeks add a fixed number of calendar days. Months and years shift the calendar month/year field directly, so JavaScript's own date engine resolves any resulting overflow (e.g. adding 1 month to March 31 rolls to May 1, since April has no 31st -- a real edge case worth checking manually for month-based date math).
Start date January 30, 2026, add 5 days: the result is February 4, 2026. Subtract 1 year from June 15, 2026: the result is June 15, 2025.
For business-day-aware date math that skips weekends, see the Add Business Days Calculator instead. To go the other direction -- finding the number of days between two known dates -- use the Days Until Date Calculator.
Month-based math is where results surprise people, so it helps to see a few cases. Take the start date March 31, 2026 and add 1 month. April has only 30 days, so the tool uses the month-end rule (the same one Excel's EDATE uses): the day is clamped to the last day of the target month and the result is April 30, 2026. Now subtract 1 month from March 31, 2026: February 2026 has 28 days, so the result is February 28, 2026. Note that the rule is applied to the start date each time, so adding 1 month and then another month is not always the same as adding 2 months in one step (March 31 + 2 months is May 31).
Weeks and days are steadier. Adding 12 weeks to a Monday always lands on a Monday, because 12 weeks is exactly 84 days. Adding 90 days to a date usually does not land on the same weekday, since 90 is not a multiple of 7. The result panel shows the weekday, which makes this easy to verify.
Leap years create a special case. If you start from February 29 of a leap year and add 1 year, the following year has no February 29, so the tool gives February 28. Subtracting a year from March 1 of a non-leap year gives March 1 of the prior year, not February 29. When a legal or financial date depends on this, confirm the convention in your agreement.
Different documents count days differently. Some exclude the start day, others include it, and some deadlines that fall on a weekend or holiday move to the next business day. This tool adds a straight calendar interval, so it does not apply those rules. For contracts, court deadlines, tax filings, or notice periods, read the exact wording and, when the stakes are high, confirm with a lawyer or the relevant agency.
Yes -- all date math uses the calendar's actual leap-year rules, so February always has the correct number of days for the year in question.
JavaScript's date engine rolls the date forward into the next month (e.g. January 31 + 1 month becomes March 3, not February 31) -- if that's not the behavior you want, double-check month-based results manually.
No -- enter a whole number and choose the smaller unit instead (e.g. 17 days instead of 2.5 weeks) for an exact result.
If the target month is shorter than the start day, the tool clamps to the last day of that month. For example, adding a month to March 31 gives April 30 because April has 30 days.
The calculation works with calendar dates, so it should not change across time zones, but confirm the date shown matches what you entered, especially near midnight.
Run it in two steps. Add the month first, then use that result as the new start date and add the days. The order can affect the outcome around month ends.
The result is simply the start date shifted by the chosen amount, so the start date is not counted as day one. If your agreement counts it as day one, subtract one day.
Reference: U.S. Department of Labor