Date Calculator

Date arithmetic looks trivial and contains three traps. All three come from the calendar being irregular rather than from the arithmetic being hard.

The first is that months have different lengths, so "one month later" is not a fixed interval. What should one month after January 31 be? The convention every date library and every bank follows is to clamp: the answer is February 28, or 29 in a leap year. Adding 31 days instead gives March 3, and a naive implementation that sets the month field and leaves the day alone also gives March 3, because the underlying date object rolls the overflow forward. Clamping is what matches how leases, billing cycles and loan payments behave.

The second is that month arithmetic does not commute with day arithmetic. From January 31, adding a month and then subtracting a day gives February 27; subtracting a day and then adding a month gives January 30. Neither is wrong; they are different operations. The convention is to apply the coarse units first, years then months then days, which is what this calculator does.

The third is that a difference between two dates has two correct answers that cannot be derived from one another. From January 1 to October 7, 2026 is 279 days. It is also 9 months and 6 days. You cannot get the second from the first without knowing the actual calendar, because the months in between were 28, 31, 30, 31, 30, 31, 31, 30 and 30 days long. The calendar difference is computed by counting whole months forward from the start date and then measuring the leftover days. That is also the only way to guarantee the day component never comes out negative.

Business days add their own convention. Counting weekdays between two dates is straightforward: full weeks contribute five each, and the remainder is walked day by day. Holidays are not. They differ by country, by state or province, by industry and by employer, and a calculator that silently assumed US federal holidays would be wrong for most of the world. This calculator counts weekdays and says so, leaving holidays to be deducted manually. Note also the convention difference: a plain day count is exclusive of the start date, while a business-day count for project planning normally includes both ends.

Two implementation details worth knowing because they are where date bugs live.

Everything here is computed in UTC. A date calculator using local time gives different answers depending on where it runs, and the difference shows up as an off-by-one day for anybody in a different time zone from the server. Since these pages are rendered once and read everywhere, local time would be wrong for most readers.

And invalid dates are rejected rather than coerced. Asked for 2026-02-30, the standard JavaScript date constructor silently returns March 2. This calculator returns an error, because a date calculator that accepts a date that does not exist will give a confident wrong answer rather than no answer.

A last piece of calendar trivia that is occasionally load-bearing: leap years are every year divisible by 4, except centuries, except centuries divisible by 400. So 2000 was a leap year and 1900 was not, which is a distinction a surprising amount of software got wrong in the late 1990s.