Check whether a local time exists on a DST day

When the clocks go forward, some wall-clock times do not exist. When they go back, some happen twice. classifyLocal tells you which case you have before you pick an instant.

import { classifyLocal } from "@northguild/gmt/instant/convert";

classifyLocal("2024-03-31T02:30:00", "Europe/Berlin"); // "nonexistent"
classifyLocal("2024-10-27T02:30:00", "Europe/Berlin"); // "ambiguous"
classifyLocal("2024-03-31T12:30:00", "Europe/Berlin"); // "unique"

On 2024-03-31 Berlin moves from 02:00 to 03:00, so 02:30 is skipped. On 2024-10-27 it moves from 03:00 back to 02:00, so 02:30 happens twice.

Other zones

Not every change is an hour, and not every change is at 02:00.

import { classifyLocal } from "@northguild/gmt/instant/convert";

classifyLocal("2024-10-06T02:15:00", "Australia/Lord_Howe"); // "nonexistent"
classifyLocal("2024-04-07T01:45:00", "Australia/Lord_Howe"); // "ambiguous"
classifyLocal("2024-09-08T00:30:00", "America/Santiago"); // "nonexistent"
classifyLocal("2024-03-31T02:30:00", "Asia/Tokyo"); // "unique"

Australia/Lord_Howe shifts by 30 minutes. America/Santiago skips midnight itself, so 00:30 does not exist on 2024-09-08. Asia/Tokyo has no clock changes.

Resolve it with a rule

resolveLocal returns a UTC instant. The default is compatible: a time in a gap moves forward by the length of the gap, and a time in an overlap takes the earlier instant.

import { resolveLocal } from "@northguild/gmt/instant/convert";

resolveLocal("2024-03-31T02:30:00", "Europe/Berlin"); // "2024-03-31T01:30:00Z"
resolveLocal("2024-10-27T02:30:00", "Europe/Berlin"); // "2024-10-27T00:30:00Z"

Pass disambiguation to choose another rule.

import { resolveLocal } from "@northguild/gmt/instant/convert";

resolveLocal("2024-10-27T02:30:00", "Europe/Berlin", { disambiguation: "later" }); // "2024-10-27T01:30:00Z"
resolveLocal("2024-03-31T02:30:00", "Europe/Berlin", { disambiguation: "reject" }); // ""

reject returns "" for a time that does not exist or is repeated, so you can handle it as an error. The four values, compatible, earlier, later and reject, are the ones Temporal uses (Temporal 13.7).

What to expect

Wall-clock time	Zone	classifyLocal	resolveLocal
2024-03-31T02:30:00	Europe/Berlin	"nonexistent"	"2024-03-31T01:30:00Z"
2024-10-27T02:30:00	Europe/Berlin	"ambiguous"	"2024-10-27T00:30:00Z", or "2024-10-27T01:30:00Z" with later
2024-10-06T02:15:00	Australia/Lord_Howe	"nonexistent"	not shown
2024-09-08T00:30:00	America/Santiago	"nonexistent"	not shown
2024-03-31T02:30:00	Asia/Tokyo	"unique"	not shown

Checked on 2026-10-26 with GMT 1.20.0.

See also: classifyLocal, resolveLocal, and the guide DST Disambiguation.