How days between dates are counted
Each date is converted to a day number and the two are subtracted. Because the calculation uses calendar dates rather than clock times, daylight saving changes cannot introduce off-by-one errors.
Days in each month
| 31 days | 30 days | 28 or 29 days |
|---|---|---|
| Jan, Mar, May, Jul, Aug, Oct, Dec | Apr, Jun, Sep, Nov | Feb |
Common uses: countdowns to holidays, trips and weddings; deadlines and notice periods; loan and rental periods; tracking streaks and anniversaries.
How to use the calculator
- Enter the start date and the end date. The order doesn’t matter; if the end is earlier, the difference is shown as a negative period.
- Choose whether to include the end date in the count (useful for events and rentals that include both days).
- Read the difference in days, weeks and days, months and days, and years, months and days.
Inclusive vs. exclusive counting
By default the calculator counts the days between two dates: from Monday to Wednesday is 2 days. Some situations count both the first and last day: a hotel booking from Monday to Wednesday is 2 nights, but a conference running Monday to Wednesday lasts 3 days. If you are counting days of an event, a leave period or a contract term that includes both dates, add one day — the “include end date” option does this.
Why “months” can be ambiguous
Months have 28 to 31 days, so “one month” is not a fixed number of days. The calculator counts whole calendar months first, then remaining days. From January 31 to March 1 is 1 month and 1 day in a non-leap year (January 31 → February 28 is treated as a full month because February has no 31st). Different programs make different choices at month ends, so if a contract depends on the exact number of months, read how it defines a month.
Useful facts
- A common year has 365 days (52 weeks + 1 day); a leap year has 366.
- Leap years are divisible by 4, except century years not divisible by 400 — 2000 was a leap year, 2100 won’t be.
- The average Gregorian year is 365.2425 days, and the average month 30.44 days.
- Spreadsheets store dates as serial numbers, so subtracting two dates gives the number of days; Excel’s DATEDIF function gives years, months or days.
Common uses
- Counting down to a wedding, holiday or deadline.
- Days of service for employment, warranties and notice periods.
- Interest calculations that use actual days (actual/365 or actual/360).
- Tracking streaks, project durations and how long ago something happened.
To exclude weekends and holidays, use the business days calculator; to find someone’s exact age, use the age calculator.