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
Calendar days or business days?▾
Calendar days. For business days (excluding weekends), use the Working Days Calculator. US contracts often specify calendar vs business explicitly; the IRS, for instance, has specific rules about 'next business day' when deadlines fall on weekends or federal holidays.
What about month-end edge cases?▾
Adding 1 month to Jan 31 gives Feb 28 (or Feb 29 leap year) — the calculator clamps to the last valid day of the target month. This matches what date-fns, dayjs, and the Java Time API do. Some systems advance to Mar 3 instead; ours uses the more common clamping behavior.
Can I subtract dates?▾
Yes — enter a negative amount to go backwards. 'Start date − 30 days' is useful for working backwards from a known deadline to find when work needs to begin.
Does this handle US federal holidays?▾
No — pure calendar arithmetic. If your deadline is contractually shifted to the next business day when it falls on a federal holiday, you'll need to apply that shift yourself. SCOTUS-rule (and most federal court rules) deadlines generally roll forward to the next business day for procedural deadlines.
Can I use this for tax filing deadlines?▾
For initial calculation, yes. But: tax deadlines have specific 'mailbox rule' provisions, holiday extensions, and weekend rollovers that are jurisdiction-specific. For a binding tax deadline calculation, IRS Publication 17 or your state department of revenue is authoritative.
What if I want to add fractional months (like 1.5 months)?▾
The calculator works in whole units. For 1.5 months, calculate 1 month + 15 days separately, or just enter 45 days. The unit dropdown only accepts whole-month and whole-year additions to avoid the ambiguity of 'half a month'.