Date calculator
Find the date a number of days, weeks, months or years from a start date.
Fill in the fields and the result will appear here automatically.
Use this date calculator to add an interval to a date or subtract one from it. It answers questions like "what date is 90 days from today" and also reports the weekday, the total shift in calendar days, the day of the year and the ISO 8601 week number.
How it works
Formula and logic
Interval components must be non-negative whole numbers; a separate switch supplies the direction. Years and months combine into one month shift, clamping the day to the end of the target month; only then are weeks ×7 + days added or subtracted. A blank component means 0. A calendar month is not a fixed 30 days, and clamping can make the operation irreversible. A result beyond the supported calendar produces an error rather than a partial date. Weekday, day of year and ISO week describe the resulting date. This is calendar arithmetic without holidays, time zones or automatic legal deadline rules. Dates use YYYY-MM-DD and years 0001–9999 under the proleptic Gregorian calendar, without country-specific historical changeovers.
Example
1 January 2026 plus 90 days is 1 April 2026, a Wednesday — a shift of 90 calendar days, the 91st day of the year, ISO week 14.
Fields and units
- Start date — date. Use YYYY-MM-DD, years 0001–9999, in the proleptic Gregorian calendar.
- Direction — Add to the date / Subtract from the date
- Years — years. Non-negative whole number; blank means 0. Select the direction separately.
- Months — months. Non-negative whole number; blank means 0. Select the direction separately.
- Weeks — weeks. Non-negative whole number; blank means 0. Select the direction separately.
- Days — days. Non-negative whole number; blank means 0. Select the direction separately.
How to use
- — Pick the start date — today is filled in by default.
- — Choose whether to add the interval to the date or subtract it.
- — Fill in the parts of the interval you need: years, months, weeks and days can be combined.
- — Check the resulting date, its weekday and the total shift in calendar days.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- NASA, section 2.1: Gregorian leap rule and proleptic dates; anniversary and shift conventions belong to this model
- Limitation
- For legal deadlines, check how dates are counted in the relevant contract, law or local working calendar.
FAQ
How are months counted when they have different lengths?
Months are added by the calendar, not as 30 days. If the target month has no such day, the last day of that month is used: 31 January plus one month gives 28 February.
Why does adding a month and then subtracting one not always return the original date?
Because of clamping. 31 January plus a month is 28 February, and 28 February minus a month is 28 January. That is normal calendar arithmetic, not a rounding error.
What happens when the start date is 29 February?
The day is clamped only when the target month lacks that day. A move into February of a common year gives 28 February, but 29 February 2024 + 13 months gives 29 March 2025 because March has a day 29. Days and weeks are applied after the month shift.
Does it skip weekends and holidays?
No, it counts consecutive calendar days. Use the business days calculator when you need working days only.
What is the ISO 8601 week number?
It is the international standard for numbering weeks: a week starts on Monday, and the first week of a year is the one containing the first Thursday. That is why 1 January sometimes belongs to the last week of the previous year.
Can I combine years, months and days?
Yes. Years and months are applied first, then weeks and days. The order matters whenever clamping to the end of a month happens.