How to count the days between two dates

Does the end date count?

The usual spreadsheet rule is end date minus start date, so the last day is not counted. From 1 January to 31 January 2027 that gives 30 days. If both days count — a leave of absence, a hotel stay measured in days, an event that runs from the first to the last day — add one: 31 days. Always check which rule a contract, form or school deadline uses.

Business days

Business days are Monday to Friday. Public holidays differ by country and year, so a simple calculator does not remove them; subtract them yourself. As an example, 10 business days after Monday 1 February 2027 is Monday 15 February 2027, before taking any holidays into account.

Adding days, weeks and months

Adding days or weeks is exact: 90 days after 15 March 2027 is 13 June 2027. Months are trickier because they have different lengths. When the target month is shorter, the date moves to its last day, so 31 January 2027 plus one month is 28 February 2027, while in the leap year 2028 it is 29 February.

Weeks, months and years

A span can be shown as total days, as weeks and days, or as years, months and days. From 1 January to 31 December 2027 is 364 days, which is exactly 52 weeks, or 11 months and 30 days. Use days for deadlines and the years-months-days form for ages and anniversaries.

Common uses and mistakes

Typical jobs are contract and notice periods, project schedules, countdowns, warranty and refund windows. Mistakes to avoid: forgetting whether the end date counts, assuming every month has 30 days, and forgetting local public holidays when counting business days.

Tools used in this guide