Date Add / Subtract Calculator
Add or subtract years, months, weeks, or days from any date to find the resulting calendar date.
What is Date Add / Subtract Calculator?
The date add / subtract calculator shifts any date forward or backward by a given number of years, months, weeks, and days. Handy for contract end dates, project milestones, warranty expirations, pregnancy due dates, and so on.
Formula
Years and months are added to the respective date components first, then weeks × 7 + days are added to the day component. JavaScript's Date object automatically rolls over month/year boundaries (adding 35 days to Jan 15 correctly gives Feb 19).
Worked example
Starting from Apr 20, 2026, add 0 years, 3 months, 2 weeks, 5 days:
- Month moves from April to July → Jul 20, 2026
- Plus 14 + 5 = 19 days → Aug 8, 2026
How to use this calculator
- Pick a starting date.
- Choose to add or subtract.
- Enter any combination of years, months, weeks, and days.
- The resulting date, day of the week, and ISO form appear instantly.
Frequently asked questions
What happens when adding months crosses a different-length month?
JavaScript clamps to the end of the month. For example, Jan 31 + 1 month becomes Feb 28 (or 29 in a leap year), since Feb 31 does not exist. This matches the common-sense convention most legal and financial documents use.
Can I use negative values?
The input fields are non-negative; use the operation dropdown to choose add or subtract. This avoids sign confusion.
What time zone does it use?
The calculation is timezone-independent because it operates on calendar components (year, month, day), not wall-clock time. So the result is the same calendar date in any zone.
Is this exclusive or inclusive of the start date?
Exclusive — the result is the date that is the specified interval after the start. Adding 1 day to Jan 1 gives Jan 2, not Jan 1.