How It Works
The Date Add/Subtract Calculator takes a start date and a duration — any combination of years, months, weeks, and days — and returns the exact resulting date.
Select Add to move forward in time or Subtract to move backward. Fill in at least one duration field and click Calculate.
The Formula
Duration is applied in two stages:
- Years and months first — using calendar arithmetic. The year and month are adjusted together, then the day is clamped to the last valid day of the resulting month (for example, January 31 + 1 month → February 28 or 29 in a leap year)
- Weeks and days second — applied as exact 24-hour increments using millisecond arithmetic
This two-stage order matches how dates are handled in contracts, billing cycles, and official documents, where “one month from January 31” means the last day of February — not 31 days later.
Common Uses
- Subscription and contract expiry — find the renewal date exactly 1 year, 6 months, or 30 days from a start date
- Medical and pregnancy tracking — add a number of weeks to a reference date such as a gestational age or recovery start
- Legal notice periods — subtract a required number of days from a deadline to find the last filing date
- Project scheduling — determine what date falls N weeks or months from a kickoff date
- Financial calculations — compute maturity dates, grace periods, and payment deadlines
Understanding the Result
The result shows the full date including the day of the week — so you can immediately see if it falls on a weekend or public holiday.
The subtitle confirms the total number of days between your start date and the result, giving a quick sanity check.
Need to find the number of days between two specific dates? Try our Date Calculator.