Date Calculator
Add or subtract days from a date, or calculate the exact number of days between any two dates.
How to use the Date Calculator
- Enter your inputs into the Date Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The date arithmetic
New date = Start date + offset days · · · Difference = (End − Start) measured in calendar days
The calculator handles month lengths, leap years, and the Gregorian calendar transition correctly. Time zones are ignored — all calculations are calendar-date based.
Worked example
2026-01-15 plus 100 days = 2026-04-25. Days between 2024-12-25 and 2026-05-26: 517 days. Days between 1990-03-15 and 2026-05-26: 13,222 days.
Frequently asked questions
Are leap years handled?
Yes. Leap years (every 4 years, except century years not divisible by 400) add a Feb 29. The calculator accounts for this correctly in all calendar arithmetic.
Does this include both start and end dates?
For "days between," the calculator returns the count of full days between, exclusive of one endpoint. To include both endpoints, add 1 to the result.
What's the date format?
ISO 8601 (YYYY-MM-DD) is unambiguous and used internally. The interface may accept multiple formats; the displayed result is always unambiguous.