How Date Calculations Work
Calculating the time between two calendar dates involves reconciling variations in month lengths (28, 29, 30, or 31 days) as well as leap years.
This calculator provides both precise total day counts and natural human-readable calendar breakdowns (years, months, weeks, and days). It also breaks down elapsed intervals into workdays (Monday through Friday) and weekends (Saturday and Sunday).
Need to count business days with holidays excluded, or add a number of working days to a date? Use the Days Calculator, which skips weekends and US federal holidays.
Frequently Asked Questions
Why is there an option to "include end day"?
In everyday language, saying "from Monday to Tuesday" can either mean 1 full elapsed day (24 hours between them) or 2 calendar days when both boundary dates are counted (e.g., event passes or hotel day-use). Toggling this setting adds exactly 1 day to the total count.
How are leap years handled?
Under the Gregorian calendar system, years evenly divisible by 4 are leap years, except for end-of-century years which must also be divisible by 400. All calculations account for the extra February 29 leap day automatically.
Can I calculate forward or backward from an existing date?
Yes, switch to the "Add / Subtract Days" tab to add or subtract any arbitrary combination of years, months, weeks, and days from any starting date.
What's changed
- Launched.
References
- U.S. Naval Observatory (USNO) — Astronomical & Calendar Systems
the Gregorian leap year rule: every year divisible by 4 is a leap year, except century years, which must also be divisible by 400