Date Calculator (Add or Subtract Time)
What's 90 days from today? What was the date 6 weeks ago? Add or subtract any number of days, weeks, months or years from a date — with month-end quirks handled correctly.
How to use this calculator
- Pick a start date (or leave empty for today).
- Choose add or subtract, the amount and the unit.
- Press the button — the result shows the weekday too, so you know what you're landing on.
- Month math clamps sensibly: Jan 31 + 1 month = Feb 28/29, not March 3.
Formula used
Days and weeks are exact. Months and years move the calendar month/year and keep the day-of-month, clamping to the target month's last day when needed — the standard convention for expiry dates and anniversaries.
Example calculation
Today + 90 days: the classic visa/returns/probation window — from 25 July that lands on 23 October.
Edge case done right: 31 January + 1 month = 28 February (29 in leap years), because February has no 31st.
Adding time to dates, correctly
"90 days" and "3 months" are not the same thing — three calendar months from 25 July is 25 October (92 days). Contracts, visas and warranties specify one or the other, and the difference regularly matters at the boundaries.
Month arithmetic also needs a rule for missing days: a month after 31 January can't be 31 February, so the convention clamps to the month's last day. Credit cards, subscriptions and legal deadlines all follow this clamping behavior, which this calculator reproduces exactly.
Why use this calculator?
- Compute expiry dates, return windows and notice periods without counting on your fingers.
- See the weekday of the result — deadlines on weekends need earlier action.
- Get month-end edge cases (Jan 31 + 1 month) right automatically.
Frequently asked questions
What date is 90 days from today?
Leave the start date empty, keep +90 days, and press the button. Ninety days is roughly three months, but the calendar-month version can differ by a couple of days — use whichever your document specifies.
What happens when I add a month to January 31?
You get the last day of February (the 28th, or 29th in leap years). When the target month is shorter, the date clamps to its final day — the standard convention in finance and law.
How do I count backwards from a deadline?
Choose Subtract. If a submission is due 1 October and needs 6 weeks of work, subtracting 6 weeks tells you the latest safe start date: 20 August.