The basic idea
The number of days between two dates is the later date minus the earlier date. From 1 January 2026 to 31 December 2026 is 364 days, or exactly 52 weeks.
Include the last day or not?
Subtracting gives the gap between the dates. If you want to count both the first and the last day, add one. Decide which you need before you calculate, especially for contracts or notice periods.
Watch the leap years
A leap year has 366 days, with 29 February. Any span that includes 29 February has one extra day.
Where this helps
- Project deadlines and notice periods
- Countdowns to holidays or events
- Working out how long something has lasted
- Checking a due date or warranty period