Date Addition Calculator
Add years, months, weeks and days to a date, with month-end clamping and the weekday of the result.
Your inputs
Results
Resulting date
Saturday, February 28, 2026
- ISO date
- 2026-02-28
- Day of the week
- Saturday
- Calendar days added
- 28
✓Exact calculation — rounded only for display
Notes for this result
- Years and months are added first (clamping to the last day of shorter months), then weeks and days.
What this calculator does
Find the date a given period after a start date — for contracts, warranties, notice periods or planning.
Formula and method
Result = (start + years × 12 + months, clamped to month end) + weeks × 7 + days
Worked example
Calculated by this tool with its default inputs:
Inputs
- Start date: 2026-01-31
- Years: 0
- Months: 1
- Weeks: 0
- Days: 0
Results
- Resulting date: Saturday, February 28, 2026
- ISO date: 2026-02-28
- Day of the week: Saturday
- Calendar days added: 28
How to read the results
Adding one month to 31 January gives the last day of February.
Assumptions and limitations
- Date-only arithmetic on the Gregorian calendar; results do not depend on your time zone or daylight-saving changes.
- Order matters: months are added before days.
Questions
Why might adding 30 days differ from adding 1 month?
Months have 28–31 days; adding a month moves to the same day of the next month where possible.
Methodology and verification
- Result type
- Exact mathematical calculation (rounded only for display)
- Formula version
- 1.0
- Automated verification
- 4 reference and validation test cases, checked against independently computed values. The site cannot be built if any of them fail.
- Editorial review
- Awaiting human editorial review. The explanation was drafted with AI assistance; the formula and examples are verified by the automated tests above.
Privacy
Everything you enter is calculated in this browser and is not sent to Numerith or anyone else. Inputs are added to a share link only if you choose to include them. Privacy policy