Date differences

Package: DATETIME_PACKAGE. Registered by createEngine(); for a slimmer engine, register it explicitly (see choosing packages).

A date difference measures the span between two dates, in whichever unit you ask for, or the time until or since a single date reckoned from now.

A duration to or from now moves with the day. The answers shown for those are for noon on Wednesday 11 March 2026 in London, the fixed moment these pages are checked against, and the notepad gives yours. A count from today is in whole days, because today is a day: 289 days to 25 December, not 288.50 at noon. An app that keeps the time of day with date.relativeDays: "now" counts the part of today already gone instead (see counting from the start of the day).

weeks between 01/01/2024 and 01/06/2024 // 21.71 weeks

days until 25/12/2026 // 289 days
days since 01/01/2023 // 1,165 days

Subtracting one date from another

Taking one date away from another gives the number of days between them, which is what the question usually is: how long until the holiday, how many days a booking runs. A later date taken from an earlier one gives a negative count, and the answer converts like any length of time.

25/12/2026 - 24/12/2026 // 1 day
2026-12-25 - 2026-01-01 // 358 days
24/12/2026 - 25/12/2026 // -1 day
(2026-12-25 - 2026-01-01) in weeks // 51.14 weeks

The days are counted on the calendar, as days between counts them, so a day the clocks change in still counts as one: 31/03/2024 - 30/03/2024 is 1 day, though that day in London was 23 hours long. A difference added back to a date moves it by those days.

31/03/2024 - 30/03/2024 // 1 day
2026-12-24 + (2026-12-25 - 2026-12-24) // Friday, December 25, 2026

The boundary: when either side has a time of day, the answer is the time that passed, on a clock, since a time in the question asks for the hours.

2026-12-25 09:00 - 2026-12-24 // 33:00

Counting a weekday

fridays between two dates counts how many Fridays fall in the range, which is a different question from how many weeks it spans. Every weekday name works, in the plural a person counting would write or in the singular, and a leading how many reads the same.

fridays between 01/06/2026 and 31/08/2026 // 13
how many fridays between 01/06/2026 and 31/08/2026 // 13
mondays between 01/06/2026 and 31/08/2026 // 14
weeks between 01/06/2026 and 31/08/2026 // 13 weeks

Those three ranges are the same three months. It holds fourteen Mondays and thirteen of everything else, because 1 June 2026 and 31 August 2026 are both Mondays, which is the part weeks between cannot tell you: it answers thirteen whichever weekday you meant.

until and since count against today rather than a second date.

mondays until 25/12/2026 // 41
sundays since 01/01/2026 // 10

Both ends are included

A range written to a Friday was written to include it, so a Friday on either endpoint is counted. It follows that a single day counts as one if it is that weekday and none if it is not, and that fridays until a Friday counts today.

It counts calendar weekdays and does not consult a holiday calendar, because a Friday that is a public holiday is still a Friday. working days between is the form that skips holidays, and it already exists: see working days.