How to calculate the days between two dates

Notice periods, deadlines and durations. Counting on a calendar is slow and gets leap years wrong.

Date arithmetic is easy to get slightly wrong, and slightly wrong is enough to miss a deadline.

Step-by-step

1. Enter two dates for a duration, or one date and an interval to add or subtract.
2. Choose whether to count all days or working days.
3. Read the result in days, weeks or months.

Inclusive or exclusive

The commonest source of an off-by-one error. Monday to Friday is four days apart and five days inclusive. Contracts and legal notices usually mean inclusive counting; "in five days" usually means exclusive.

When a deadline matters, work out which convention applies before you rely on the number.

Working days

Working-day counts here exclude weekends. They do not know your public holidays, which vary by country and often by region, so check the result against a holiday calendar for anything with a legal consequence.

Months are not a fixed length

Adding one month to 31 January has no unambiguous answer, and different systems resolve it differently — 28 February, 2 March, or an error. Adding a month then subtracting one does not always return you to where you started.

For anything precise, count in days. Months are convenient for people and unreliable for arithmetic.

Frequently asked questions

Should I count the first day?

It depends on the convention in use. Legal notice periods are usually inclusive; everyday phrasing like 'in five days' usually is not. Establish which applies before relying on the figure.

Does the working-day count exclude public holidays?

No, only weekends. Holidays vary by country and region, so check against the relevant calendar for anything with a deadline attached.

How is adding a month handled at the end of a month?

Month arithmetic is inherently ambiguous at month ends. Where precision matters, count in days instead.