Date Calculator
A date, an amount, and a unit.
Days are simple. Months are not.
Adding days or weeks is arithmetic: a fixed number of 24-hour periods, and the answer is never ambiguous.
Adding months is a different operation entirely, because months are not a fixed length. "One month after 31 January" has no correct answer. February has no 31st.
What happens instead
The convention almost all software follows — and what happens here — is to clamp to the end of the month. One month after 31 January becomes 28 February, or 29 in a leap year. The tool says so explicitly when it happens, rather than quietly producing a date you did not expect.
The consequence is that month arithmetic is not reversible. Add a month to 31 January to get 28 February, subtract a month, and you get 28 January — not where you started. If a contract depends on this, read how it defines the date rather than assuming.
Years have the same problem, once every four
One year after 29 February 2024 is not a date that exists in 2025. It rolls to 1 March, which is flagged above when it happens.
The 30-day month is a fiction worth knowing about
Some financial calculations deliberately assume every month is 30 days and every year 360 — the "30/360" convention used in bond markets. It makes interest periods uniform and it does not match the calendar. If a figure from a financial system disagrees with a real date calculation, this is often why.
Common uses
- Deadlines — 30 days from an invoice, 14 days from a notice.
- Renewal and expiry dates.
- Working backwards from a fixed date to find when to start.
- Appointment and follow-up scheduling.
The working, step by step
Questions people ask
What is one month after 31 January?
28 February, or 29 in a leap year. The date is clamped to the end of the month because 31 February does not exist.
Is adding a month reversible?
No. Add a month to 31 January to get 28 February, subtract one, and you land on 28 January rather than where you started.
What is one year after 29 February?
1 March in a non-leap year, since the date does not exist.
Should I use days or months for a deadline?
Days, if the exact count matters. '30 days' is unambiguous; 'one month' depends on which month.
What is the 30/360 convention?
A financial assumption that every month has 30 days and every year 360. It makes interest periods uniform and does not match the calendar.
Worked out in the page
The arithmetic runs in your browser. No date you type is sent anywhere, which matters for the ones that are nobody else's business — a birth date, a due date, a notice period. There is no server here to receive them. The about page lists every request the site makes.