About the Date Calculator
A two-mode date calculator. Mode one: 'days between' — pick two dates and see how many days apart they are. Mode two: 'date offset' — pick a starting date and add or subtract a number of days to find the resulting date. Useful for working out anniversaries, project deadlines, contract terms, gestational dates, sports records, and any 'what was the date 100 days ago / 100 days from now' question.
How to use it
- Choose 'days between' or 'add / subtract days' from the mode toggle.
- In 'days between' mode, pick a start date and an end date — the count of days appears beneath.
- In 'add / subtract' mode, pick a start date and type a positive or negative number of days.
- All calculations are inclusive in a simple subtraction sense — 'days between Mon and Wed' is 2.
Useful day counts
| 1 week | 7 days |
| 2 weeks (fortnight) | 14 days |
| 1 month (avg) | ≈ 30.44 days |
| 1 quarter | ≈ 91 days |
| 1 year (non-leap) | 365 days |
| 1 year (leap) | 366 days |
| 18 years (US legal adult) | ≈ 6,575 days |
| 21 years | ≈ 7,670 days |
| Gestation (full term) | ≈ 280 days from LMP |
Frequently asked questions
- How does the calculator handle leap years?
- It uses the standard Gregorian leap-year rule: divisible by 4 is a leap year, except divisible by 100 (not a leap year), except divisible by 400 (is a leap year). So 2000 was a leap year, 1900 wasn't, 2100 won't be. The calculator works for any date Gregorian dates support.
- What about time zones?
- The calculator works in pure calendar days — it doesn't apply time zones or daylight saving. 'Days between' two dates is the same number no matter where you are.
- Can I include or exclude the end date?
- The result is a simple end − start difference. If you need 'inclusive of both endpoints' (a common contract convention), add one to the displayed result. If you need 'business days only', that's a different calculation not currently supported.
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