Two methods
- Exclusive: from the start of the first day to the start of the last. 1 → 2 January = 1 day. This is how most calculators and ages work.
- Inclusive: counts both the first and last day. 1 → 2 January = 2 days.
The difference is always one day. Ask yourself: am I measuring how much time passed, or counting days that are spent?
| Situation | Method |
|---|---|
| Time since an event | Exclusive |
| A person’s age, a contract term | Exclusive |
| Leave from Sunday to Thursday | Inclusive |
| Office days in a full month | Inclusive |
Examples
| Period | Exclusive | Inclusive |
|---|---|---|
| 1 Jan 2026 → 31 Dec 2026 | 364 days | 365 days |
| 1 Sep 2026 → 30 Sep 2026 | 29 days | 30 days |
| 1 Jan 2000 → 1 Jan 2026 | 9,497 days | 9,498 days |
Counting by hand
Add the days left in the first month, the full months in between and the day of the last date. Example: 20 January → 15 March 2026 = (31 − 20) + 28 + 15 = 54 days (exclusive). Add one for inclusive counting.
Common mistakes
- Forgetting leap years: a span containing 29 February is one day longer.
- Treating months as 30 days: “three months” is 89 to 92 days.
- Mixing calendars: convert Hijri dates first, or use a tool that accepts both.
- Counting weekends as working days: use the Working Days Calculator.
Tools: Date Difference (with an include-end-date option), Working Days, and the countdown.
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