How to calculate the number of days between two dates

Counting the number of days between two dates sounds simple until you actually try it on your fingers. Months have different lengths, some years slip in an extra day for February, and it is easy to lose track of whether you should count the first day, the last day, or both. Get any of those wrong and your answer drifts by a day or two, which matters a lot when you are working out a deadline, a notice period, or how long is left on a subscription. This guide shows you a reliable way to count days by hand, explains the traps that throw people off, and points you to a tool that does it instantly.

The manual method that actually works

There are two hand methods. The first is to count full months and then the leftover days, but that gets fiddly fast because a "month" can be 28, 29, 30, or 31 days. The more reliable approach is to convert each date into a plain day number, then subtract. To get a day number, add up the lengths of every month before it in the year and add the day of the month.

Say you want the days from January 20, 2025 to March 10, 2025. Turn each date into its position in the year and subtract:

Start: Jan 20, 2025 = day 20 of the year

End: Mar 10, 2025 = 31 (Jan) + 28 (Feb) + 10 = day 69

Days between = 69 - 20 = 49 days

You can double-check that by walking it out: 11 days left in January (31 minus 20), plus all 28 days of February, plus 10 days into March, which is 11 + 28 + 10 = 49. Both routes agree. If your two dates fall in different years, count the days to the end of the first year, add the day number in the second year, and remember that a full year adds 365 days, or 366 if a February 29 sits inside the span.

Inclusive versus exclusive counting

This is the single biggest reason two people get different answers from the same pair of dates. Exclusive counting measures the gap between the dates, the way hotel nights work. Inclusive counting counts both endpoints as full days, the way you might count the days you are away on a trip.

Picture a hotel stay: you check in on June 10 and check out on June 13. The gap is 13 - 10 = 3 nights, which is what you pay for. But you actually touch four different calendar dates: the 10th, 11th, 12th, and 13th. So the exclusive count is 3 and the inclusive count is 4. The plain subtraction method above always gives you the exclusive gap. If you need both endpoints counted, add one to the result.

Do not forget leap years

February normally has 28 days, but in a leap year it has 29. That single extra day quietly changes any span that crosses it. The rule: a year is a leap year if it is divisible by 4, except for century years, which must be divisible by 400. So 2024 is a leap year and 2028 will be, but 1900 was not and 2100 will not be, while 2000 was because 400 divides it.

Here is the effect in practice. The days from February 1 to March 1 come out to 28 in 2025, a normal year, but 29 in 2024, a leap year. A full span from one date to the same date a year later is 365 days most of the time, and 366 whenever February 29 falls inside it. Miss the leap day and every calculation that reaches across it is off by one.

Calendar days versus business days

A calendar day count includes every day, weekends included. A business day count skips Saturdays and Sundays, and often public holidays too. The difference adds up quickly. One calendar week is 7 days but only 5 business days. Two calendar weeks are 14 days but 10 business days.

Contracts, shipping estimates, and bank processing times are frequently quoted in business days, so "pays out in 5 business days" starting on a Wednesday actually lands on the following Wednesday once the weekend is stepped over. If holidays fall inside the window, push it out further. When someone gives you a day count, it is always worth confirming whether they mean calendar days or working days.

Common durations at a glance

Most everyday questions come down to a handful of familiar spans. The table below lists the ones people ask about most, in exact days and a rough equivalent, so you can sanity-check a figure without counting.

A few of these are worth committing to memory. Since 52 weeks is 364 days, a regular year runs one day over a neat 52 weeks and a leap year runs two days over, which is why the same calendar date drifts forward by a day or two each year. And because a month averages just over 30 days, 90 days is close to but not exactly three calendar months, depending on which months you land on.

Frequently asked questions

How do I count the days between two dates by hand?

Convert each date into its day number in the year by adding up the lengths of the months before it and adding the day of the month, then subtract the smaller number from the larger. For dates in different years, count to the end of the first year, add the day number in the second, and add 365 for each full year in between, or 366 if a February 29 falls inside the span.

Should I count both the start and end dates?

That depends on whether you want an inclusive or exclusive count. Plain subtraction gives the exclusive gap, like nights in a hotel. If you need both the first and last day counted as full days, such as the length of an event, add one to the result.

How do leap years change the count?

A leap year adds one extra day, February 29, so any span that crosses it gains a day. A year is a leap year when it divides evenly by 4, except century years, which must divide by 400. That makes a normal year 365 days and a leap year 366.

What is the difference between calendar days and business days?

Calendar days count every day of the week, while business days skip weekends and usually public holidays. One calendar week is 7 days but only 5 business days, so a deadline quoted in business days always lands later on the calendar than the same number of calendar days.