# Check whether a local time exists on a DST day

Industry: Core

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.

```js
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.

```js
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.

```js
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.

```js
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`](/gmt/reference/instant/convert/classifyLocal/), [`resolveLocal`](/gmt/reference/instant/convert/resolveLocal/), and the guide [DST Disambiguation](/gmt/guides/concepts/dst-disambiguation/).