Add or subtract from a date
Work out the date a number of days, weeks, months or years from a start date — with correct month-end handling.
Runs in your browser
What this answers
Pick a start date, a number and a unit, and it gives you the date. 90 days from today, six months before a deadline, two years after a contract started.
The result is shown with its day of the week, which is usually the second thing you want to know and always the thing you forget to check.
Adding months is not adding 30 days
The interesting case, and the one that makes date libraries disagree with each other:
What is one month after 31 January?
| Answer | Reasoning |
|---|---|
| 28 February | clamp to the last day of the target month |
| 3 March | add one to the month, let the day overflow |
| 2 March | add 30 days |
This page gives 28 February — clamping — which is what almost every calendar application, and most legal drafting, means by “one month later”. In a leap year it gives 29 February.
The consequence is that month arithmetic is not reversible. One month after 31 January is 28 February; one month before 28 February is 28 January, not the 31st you started from. That is not a bug in the calculator, it is a property of the calendar, and it is why contracts that matter say “on the last day of the month” rather than relying on the arithmetic.
| From | Plus one month |
|---|---|
| 31 January | 28 February (29 in a leap year) |
| 31 March | 30 April |
| 30 January | 28 February |
| 29 February 2024 | 29 March 2024 |
Adding years to 29 February
The same clamp applies. One year after 29 February 2024 is 28 February 2025; four years after is 29 February 2028.
Which unit to use
They are not interchangeable, and picking the wrong one is the commonest error here:
| Written as | Use | Because |
|---|---|---|
| “90 days” | days | a fixed count, unaffected by month lengths |
| “3 months” | months | lands on the same day number, 89–92 days later |
| “6 weeks” | weeks | lands on the same weekday |
| “1 year” | years | same date, next year |
90 days and 3 months are different periods. From 1 January, 90 days is 1 April and three months is 1 April — they agree by coincidence. From 1 March, 90 days is 30 May and three months is 1 June. Which one applies depends entirely on the wording of the rule.
Weeks are the only unit that guarantees the same weekday, which is why rotas and recurring appointments are specified in weeks.
Business days are not offered
Deliberately. Adding “10 working days” requires a holiday calendar, and holidays are jurisdictional and move every year — Lunar New Year, Easter, and every substitute weekday that follows a holiday falling on a weekend. A generic calculator that claimed to do it would be quietly wrong for most people using it.
The difference calculator does count business days between two known dates, with the same caveat: weekdays only, no holidays.
Common periods
| Period | From 1 January 2026 |
|---|---|
| 30 days | 31 January 2026 |
| 90 days | 1 April 2026 |
| 6 months | 1 July 2026 |
| 1 year | 1 January 2027 |
| 18 months | 1 July 2027 |
Everything is calculated locally
The date never leaves your browser. See also date difference and the age calculator.