How to Count the Days Between Two Dates Without Mistakes

Working out the number of days between two dates is useful for contracts, notice periods, warranties, travel and countdowns. It is simple in principle and easy to get off by one.

Decide whether the end dates count

The first question is whether you include the start date, the end date, both or neither. Conventions differ:

• Exclusive counting (the usual default): from 1 March to 5 March is 4 days.
• Inclusive counting counts both ends: 1 March to 5 March is 5 days.

Contracts and rules often state which they mean. If a document says "within 14 days of the date of the letter", the letter's own date is normally day zero.

The manual method

1. If the dates are in the same month, subtract the day numbers.
2. If they are in different months, count the days remaining in the first month, add the full lengths of any months in between, then add the day number of the final date.
3. If the span crosses 29 February in a leap year, remember to use 29 for that month.

For example, from 18 January to 9 April in a normal year: 13 days left in January (31 − 18), 28 in February, 31 in March, then 9 in April — 81 days.

Memorising month lengths

The classic rhyme — thirty days has September, April, June and November — covers the short months, with February the exception. The knuckle method works too: knuckles are 31-day months, gaps between them are shorter ones.

Leap years

A year is a leap year if it is divisible by 4, except for century years, which must be divisible by 400. So 2000 and 2024 were leap years, but 1900 and 2100 are not. Spans that include the end of February need this check.

Business days versus calendar days

Legal and shipping deadlines sometimes count only working days, excluding weekends and public holidays. That is a separate calculation, and holidays differ by place, so check how the deadline is defined.

Common errors

• Off-by-one from mixing inclusive and exclusive counting.
• Assuming every month has 30 days.
• Reading dates in the wrong day/month order.
• Forgetting a leap day in a multi-year range.

For anything that matters, use a date tool and note which convention it applies.

Count the exact days between dates with Age Calculator →