How to Count the Days Between Two Dates

Should the end day count, how is a month difference found, and why are business days different? A practical guide for contracts, leave and deadlines.

"How many days until the deadline?", "When does the contract end?", "How long is my leave?" These look like simple questions, yet two people counting the same dates sometimes get answers one day apart. The difference is rarely a mistake. It is usually a different counting rule.

Does the end day count?

How many days are there between 1 March and 10 March? The difference between the two dates is 9 days. An event that starts on 1 March and ends on the evening of 10 March lasts 10 days, though, because both end days are included. For a hotel stay, what matters is nights, which is 9. Decide which question you are answering before you start. The "include end day" option in the date difference calculator handles exactly this.

Why month and year differences are not the same as days

Months run from 28 to 31 days, so "in 3 months" and "in 90 days" land on different dates. Cases like one month after 31 January are handled differently by different systems; some move to the last day of February. If a contract states a period in months, find the same day on the calendar rather than converting to days.

Business days are a separate count

Remove weekends and 30 calendar days become 20 to 22 business days. Public holidays change the result further, and some holidays move every year, so check that year's holiday list. You can enter holiday dates separately in the business days calculator.

Counting by hand

Within the same month, subtract the day numbers (10 − 1 = 9).
Across months, add the remaining days of the first month, the full months in between and the day number in the last month.
Remember February: it has 29 days in leap years. Years divisible by 4 are leap years, except those divisible by 100 but not by 400.
If both end days should count, add 1.

Where to be careful

For legal deadlines, such as appeals, applications or notice periods, the method of counting is defined in the relevant rules, and there may be special provisions when the last day falls on a holiday. If such a deadline is involved, confirm the result with an official source or a professional. A calculator gives you a quick starting point, not a legal answer.

The date difference calculator shows the gap in years, months and days, plus total days and weeks. To see how long is left until a date, use the time until date calculator.