Date arithmetic

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

Adding a span to a date moves it forward, and subtracting one moves it back, so you can ask what day falls twenty days after a fixed date, or three hours after now. The forms anchored to a fixed date are proven here; the ones relative to now resolve against the current date, so they compute rather than being asserted.

A day count crosses month and year boundaries; the relative forms resolve against now.

25/12/2023 + 20 days // Sunday, January 14, 2024

now + 3 hours
today - 1 week

The same sum in words

from, after and before put the span in front of the date, which is how a deadline, a notice period or an invoice term is usually written down. They are the operators above in words, so they answer the same thing.

30 days from 3 March 2026 // Thursday, April 2, 2026
2 weeks after 3 March 2026 // Tuesday, March 17, 2026
30 days before 3 March 2026 // Sunday, February 1, 2026
3 months from 3 March 2026 // Wednesday, June 3, 2026

from and after count forward and before counts back. The anchor can be any date the engine reads, including a relative one:

3 days from today

The connector is only read this way when a span is in front of it, which is what keeps $1,000 after 3 years at 7% an investment. to is deliberately not claimed at all: 2 April 2026 to 6 September 2026 already means something, and quietly turning it into an offset would take that away.

For a span that skips weekends and holidays, 30 working days from 3 March 2026 is the sibling form; see working days.