# Migrate to Temporal with GMT

GMT follows the Temporal specification. Moving to it from a date library changes the same few things whichever library you leave, so they are listed once here. Each library page then maps its calls one by one.

## What changes

- **Strings in, strings out.** A GMT value is a string in ISO 8601 form, with the RFC 9557 `[zone]` suffix for a zoned value. There is no object to construct, and a string cannot change after you make it. Check one with a validator such as `isValidDate` or `isValidZonedDateTime`.
- **Bad input gives a typed fallback.** A function that cannot read its input returns `""`, `null`, `false` or `[]` for its return type, and does not throw. A correct empty result, such as two intervals that do not overlap, is `[]` too, so read the function's page for what its fallback means.
- **Months run from 1 to 12.** This matches Temporal. A library that counts from 0 needs each month number moved by one.
- **No token formatter.** GMT formats through `Intl` with a locale and a style, as in `formatDate(value, "de-DE", { dateStyle: "long" })`. It does not read tokens such as `dd/MM/yyyy` when it writes. `parseDateWithPattern` reads them.
- **A day in a zone is a calendar day.** Adding one day to a zoned value gives the same wall-clock time on the next date when that time exists, as Temporal does. When it falls in a gap, the result moves forward by the length of the gap (Temporal 6.5.5 AddZonedDateTime, with the default `compatible` disambiguation): `addZoned("2024-03-30T02:30:00+01:00[Europe/Berlin]", { days: 1 })` gives `2024-03-31T03:30:00+02:00[Europe/Berlin]`. Adding hours adds exact time.

## Pages

| Page | What it covers |
| --- | --- |
| [date-fns](./date-fns/) | Every date-fns call with its GMT function, and the cases where date-fns differs from Temporal |
| [Luxon](./luxon/) | The same for Luxon |
| [Day.js](./dayjs/) | The same for Day.js |
| [Moment.js](./moment/) | The same for Moment.js |
| [@internationalized/date](./internationalized-date/) | The same for @internationalized/date |
| [Native Temporal and GMT](./temporal/) | Which runtimes ship Temporal, and the GMT function for each Temporal call |

The results behind the differences are in [Behaviour of date libraries against Temporal](/gmt/compare/behaviour/).

For a comparison of how each library handles time zones and DST, see [Time zones and DST in JavaScript: which date library handles them](/gmt/guides/how-to/time-zones-and-dst-in-javascript/).