The rule
When you subtract one date from another, you count the gaps between days, not the days themselves.
- Monday to Friday (19 to 23 October 2026) is 4 days apart.
- If you need to count both Monday and Friday, for example days of a trip or days worked, add 1 to get 5 days.
Most date calculators, including ours, use the first method by default and offer an “include the end date” option.
Examples
| From | To | Days apart | Counting both ends |
|---|---|---|---|
| 1 Jan 2026 | 31 Dec 2026 | 364 | 365 |
| 19 Oct 2026 (Mon) | 23 Oct 2026 (Fri) | 4 | 5 |
| 1 Feb 2026 | 1 Mar 2026 | 28 | 29 |
| 1 Feb 2028 | 1 Mar 2028 | 29 | 30 |
2028 is a leap year, so February has 29 days and the same month span is one day longer.
Counting forward
To find the date 30 days after 21 October 2026, add 30 days: you get Friday, 20 November 2026. Deadlines such as “within 30 days” can follow different rules, so check which day counts as day 1.
Working days
For notice periods and delivery times, you often need business days, which skip weekends and public holidays. Count those separately rather than taking about five-sevenths of the total.
Write dates without ambiguity
03/04/2026 is 3 April in most countries but March 4 in the US. The international standard ISO 8601 writes dates year first, as 2026-04-03, which nobody can misread.
Try it with the days between dates calculator, or work out your exact age with the age calculator.