Date Calculator
It adds or subtracts days, weeks, months or years from any date and tells you the exact date and weekday you land on.
Adding 1 months
Feb 29, 2024
Result date
Feb 29, 2024
Weekday
Thursday
Days from start
29
ISO format
2024-02-29
Day of month
29
Year
2024
What it calculates
It adds or subtracts days, weeks, months or years from any date and tells you the exact date and weekday you land on.
Why it matters
Deadlines, renewal dates, notice periods and warranty expiries are all defined as an offset from a start date, and month-end arithmetic is genuinely tricky.
Who it's for
Anyone tracking a deadline, a renewal, a due date or a statutory notice period.
Formula
- d
- Start date
- n
- Amount to add or subtract
- u
- Unit: days, weeks, months or years
Worked example
31 January 2024 plus one month
- 1February 2024 has only 29 days
- 2The 31st does not exist, so the date clamps
- 3Result: 29 February 2024
29 February 2024 — not 2 March
How the date calculator works
Days and weeks are simple addition. Months and years are not: adding a month to 31 January has no exact answer, so the result clamps to the last day of the target month rather than spilling into the next one. That matches how contracts and billing cycles are normally read, and it is why 31 January plus one month is the end of February rather than the second or third of March.
Days and weeks are simple addition. Months and years are not: adding a month to 31 January has no exact answer, so the result clamps to the last day of the target month rather than spilling into the next one.
That matches how contracts, billing cycles and statutory notice periods are normally read — which is why 31 January plus one month lands on the end of February, not the second or third of March.
Common mistakes
- Assuming a month is 30 days, which drifts across a year.
- Expecting 31 January plus one month to be 3 March; almost every legal and billing convention clamps to month end.
- Ignoring leap years when adding a year to 29 February.
Tips and best practice
- For a 30-day payment term, add 30 days rather than one month — they are different dates in most months.
- Check the weekday: a deadline landing on a weekend usually rolls to the next working day.
Frequently asked questions
What date is 90 days from today?
Set the start date to today, choose 90 days, and add. The calculator also shows the weekday, which matters for business deadlines.
What happens when I add a month to the 31st?
The result clamps to the last day of the target month. 31 January plus one month is 28 or 29 February, matching standard billing and legal convention.
Is adding one month the same as adding 30 days?
Only in a 30-day month. In January the two differ by a day, and in February by two or three.
How does it handle leap years?
Adding a year to 29 February lands on 1 March in a non-leap year, since 29 February does not exist that year.
Related calculators
Methodology & trust
- Formula source
- Calendar arithmetic with end-of-month clamping.
- Last updated
- 2026-07-28
- Privacy
- Every calculation runs in your browser. No inputs are sent to a server or stored.
- Accessibility
- Keyboard navigable, labeled inputs and WCAG AA color contrast.