Exact calendar maths
The calculator uses real calendar rules, including leap years and different month lengths, and works in UTC so daylight saving changes never shift your date by a day. It first adds or subtracts whole years and months, clamping to the last day of the month if needed, and then adds weeks and days.
Typical uses
Work out a return date, a warranty expiry, a deadline 45 days away, or a due date. For the gap between two dates use the date difference calculator, for age use the age calculator, and for pregnancy dates see the pregnancy due date calculator.
Frequently asked questions
How does adding months work when the day does not exist?
If the new month is shorter, the date is moved to the last day of that month. 31 January + 1 month = 28 February (29 in a leap year). The calculator adds years and months first, then weeks and days.
Can I subtract instead of add?
Yes. Choose Subtract and the same years, months, weeks and days are taken away from the start date.
How many days is 3 months?
It depends on the months involved, between 89 and 92 days. That is why the tool shows the exact number of calendar days between your start and result.
Does it count weekends?
Yes, every calendar day counts. To skip weekends and holidays, use the working days calculator.
How do I find the date 90 days from now?
Set the start date to today, choose Add, and enter 90 in days. The result also shows the weekday so you can plan around it.