What is a date calculator?
A date calculator finds the date that falls a given number of days, weeks, months or years before or after a starting date. It's the quick way to answer questions like "what's the date 90 days from now?" or "when was six weeks ago?" This one returns the resulting date in year-month-day format along with its day of the week.
How to use it
- Pick the start date.
- Choose Add or Subtract.
- Enter the amount as a whole number. Negative numbers aren't accepted; use Subtract instead.
- Choose the unit: days, weeks, months or years.
- Click Calculate.
The result is shown as an ISO 8601 date (2027-01-03 means 3 January 2027), which avoids the day/month mix-ups between US and European formats.
Worked examples
| Start date | Operation | Result | Day |
|---|---|---|---|
| 5 Oct 2026 | + 90 days | 2027-01-03 | Sunday |
| 5 Oct 2026 | − 6 weeks | 2026-08-24 | Monday |
| 15 Jan 2026 | + 30 days | 2026-02-14 | Saturday |
| 5 Oct 2026 | + 10 years | 2036-10-05 | Sunday |
Walking through the first one by hand shows why a calculator helps. From 5 October, there are 26 days left in October, 30 in November and 31 in December, which takes you to 31 December after 87 days. Three more days lands on 3 January 2027.
How each unit works
Days and weeks are exact. A week is always 7 days, so adding 6 weeks is the same as adding 42 days.
Months and years keep the same day number and change the month or year. 5 October plus 3 months is 5 January. That gets awkward when the target month is too short to contain the day:
| Start date | Operation | Result |
|---|---|---|
| 31 Jan 2026 | + 1 month | 2026-03-03 |
| 31 Mar 2026 | − 1 month | 2026-03-03 |
| 31 Aug 2026 | + 6 months | 2027-03-03 |
| 29 Feb 2024 | + 1 year | 2025-03-01 |
In each case the calculator first moves to the "same" day in the target month, for example 31 February, and because that date doesn't exist the extra days overflow into the next month. 2026 isn't a leap year, so 31 February 2026 becomes 3 March. This is how the underlying date library, PHP's DateTime, handles month arithmetic, and it's the case to double-check when you start on the 29th, 30th or 31st.
The second row is the surprising one: subtracting a month from 31 March gives 3 March rather than the end of February. If you need "the last day of the previous month", subtract the month from an earlier day such as the 28th, or count back in days.
Counting from day 0 or day 1
Date arithmetic counts the start date as day 0. Adding 30 days to 15 January lands on 14 February, and the 30 days are 16 January through 14 February.
Deadlines in contracts, notices and visa rules don't always work that way. Some count the day of the event as day 1, some exclude weekends, and some say "within" a period, which can end the day before. When a date matters legally or financially, read how the rule defines the period, and use the calculator's result as a check rather than the final word.
Things this calculator doesn't do
- Business days. Weekends and public holidays are counted like any other day.
- Fractional amounts. Only the whole-number part is used, so 1.9 days is treated as 1 day.
- Times of day. Dates have no time attached, so there's no time-zone or daylight-saving effect on the result.
- Differences between two dates. For that, use the age calculator, which works for any pair of dates and gives the total number of days between them.
Related calculators
- Age calculator: the years, months and days between two dates.
- Countdown calculator: how many days and hours until a date.
- Time zone converter: convert a date and time between cities.
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