isValidUtcOffset

Signature

isValidUtcOffset(value: string): boolean

Return true when value is a UTC offset in the form the offset-instant pair stores: ISO 8601 extended ±HH:MM, optionally ±HH:MM:SS.

This is the offset toOffsetInstant returns and fromOffsetInstant takes, and the one getTimeZoneOffset and getZonedOffset return.
The utcOffset pattern is the whole check. Every string it matches is in range (hour 00–23, minute and second 00–59), so this validator is the pattern test plus the answer false for a non-string.
Z is false: it is a designator meaning “this string is already UTC”, and for an event at UTC the pair’s offset is +00:00.
Seconds are allowed because some zones did not run on a whole minute: Africa/Monrovia was -00:44:30 until 1972. A fraction of a second is false.
Basic format (-0400) and an hour alone (-04) are false.
-00:00 is true, and the functions that take an offset read it as +00:00.
A function whose result has no zone in it takes this offset in its time zone position too. A function that returns a zoned string, or formats with Intl, takes a time zone identifier only: check that with isValidTimeZone, which accepts +0530 and -08 and rejects an offset with seconds.

Parameter	Type	Description
`value`	`string`	candidate UTC offset string

boolean indicating validity

import { isValidUtcOffset } from "@northguild/gmt/instant/validate";

isValidUtcOffset("-04:00") // true