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Date Calculator

Add or subtract years, months, weeks, and days from any date.

How the calculator works

The calculator applies the components in a defined order: years and months first, then weeks, then days. Month arithmetic follows calendar convention — adding one month to January 31 does not produce “February 31”; it clamps to February 28 (or 29 in a leap year), which is how legal and banking systems treat month-based terms. Weeks and days are then added as exact multiples of 7 and 1 calendar days, and subtraction simply reverses every component. The weekday is computed from the final date, so you can spot immediately whether a deadline lands on a weekend.

Formula

Result = Start ± years ∓ months ± weeks ± days
Month-end rule: min(day, days-in-target-month)

Where:

  • Start — the initial date
  • Years / months — calendar components, applied first with clamping
  • Weeks / days — exact shifts of 7 and 1 calendar days

Example

A 90-day notice served on March 15, 2026: adding 0 years, 0 months, 0 weeks and 90 days lands on June 13, 2026 — a Saturday, worth knowing before you plan a filing. For a 3-month contract extension starting November 30, the clamp rule places the end on February 28 rather than an impossible February 30. Both behaviors mirror how courts and banks count.

Frequently Asked Questions

Why does January 31 plus one month give February 28?
Because February has no 31st. Calendar arithmetic clamps the overflow to the last real day of the target month, which is the convention used in contracts and financial systems. The alternative — rolling into March 2/3 — would change the 'one month later' meaning.
In what order are the components applied?
Years, then months, then weeks, then days. The order matters in edge cases: adding 1 month then 1 day to January 30 gives March 2, while 1 day then 1 month gives March 1. This calculator uses the standard years→months→weeks→days order.
Does it handle leap years?
Yes. February 29 exists in leap years and the clamping rule automatically routes around it in common years, so a date one year after February 29, 2024 correctly lands on February 28, 2025.
Can I count backwards from a deadline?
Yes — choose “Subtract from date” and enter the same components. A useful trick for planning: start from the deadline and subtract the notice period to find the last date you can act.

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