Deadline Calculator
Add or subtract any number of days, weeks, months, or years from a start date — and see the resulting deadline.
Adding 30 days to January 1, 2026 gives January 31, 2026 — a Saturday.
Results update as you type. Months and years clamp to month-end (so Jan 31 + 1 month = Feb 28, not Mar 3) — the dominant date-library convention.
Formula
Days and weeks are simple millisecond arithmetic. Months and years use month-end clamping: adding 1 month to Jan 31 gives Feb 28 (or Feb 29 in a leap year), not March 3 — matching the dominant date-library convention. Negative amounts go backwards in time.
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Frequently asked questions
Does this count business days or calendar days?▾
Calendar days. For business-day-only deadlines (e.g. 'within 14 working days'), use the Working Days Calculator instead. UK business contracts and legal documents commonly specify either; check the wording — 'days' usually means calendar, 'working days' or 'business days' excludes weekends and bank holidays.
What happens with month-end dates like Jan 31?▾
Adding 1 month to Jan 31 gives Feb 28 (or Feb 29 in a leap year) — the calculator clamps to the last valid day of the target month. This is the dominant convention used by date libraries like date-fns, dayjs, and Java's Period. Some systems give Mar 3 instead (advancing into the next month); ours doesn't.
Negative amounts?▾
Yes — enter a negative number to go backwards. 'Start date − 14 days' tells you the date 14 days before. Useful for working out 'when did I need to start to hit this deadline?'.
How does this handle UK bank holidays?▾
It doesn't — the deadline is a pure calendar date. If your contract or process treats bank holidays specially (e.g. 'next working day' rules), you need to manually shift the deadline. The Working Days Calculator excludes weekends but currently doesn't model bank-holiday lists.
Can I use this for legal deadlines?▾
For calendar-day calculations, yes — but for legally-binding deadlines with specific wording about working days, statutory holidays, or court rules, defer to a solicitor or the official rule. England + Wales civil court rules (CPR), for instance, have specific service-of-document rules that affect deadline counting.
What if my start date is in the past?▾
Works the same way. The calculator just adds the requested amount of time — past or future starting points are equivalent arithmetically.