GMT: Give Me Temporal

The most tested date and time library for JavaScript. @northguild/gmt is one library in place of Luxon, date-fns, Moment.js, Day.js, Spacetime and @internationalized/date. It is built on the TC39 Temporal standard and never touches Date.

Docs · API reference · Tools · Discord

• 56,006 tests, run 1,680,180 times in CI. Every test runs in 10 time zones on 3 Node versions.
• 50× the CI test runs of the six libraries below, combined.
• 634 functions, to the nanosecond, tested in 17 locales.
• Never throws. ISO 8601 strings in. Invalid input returns "", null, false or [].

Library	Tests	CI test runs	Built on
@northguild/gmt	56,006	1,680,180	Temporal
Spacetime	6,086	12,172	Date
Moment.js	3,901	11,703	Date
date-fns	3,213	3,213	Date
Luxon	1,222	4,888	Date
Day.js	794	1,034	Date
@internationalized/date	386	386	own types

The other libraries were measured from 2026-08-22 to 2026-09-09, each by running its own test suite. The full comparison and the method.

Install

npm install @northguild/gmt

To ban Date in your own code, add a lint plugin: ESLint · Oxlint · Biome.

Use

import { addBusinessDays, addDate, addZoned, convertUtcToZoned, diffDate, formatDate, isValidDate, resolveLocal } from "@northguild/gmt";

addDate("2024-01-31", { months: 1 }); // "2024-02-29"
diffDate("2024-01-01", "2024-03-01", "day"); // 60
formatDate("2024-03-15", "en-GB", { dateStyle: "long" }); // "15 March 2024"
isValidDate("2023-02-29"); // false
addBusinessDays("2024-03-15", 1); // "2024-03-18"

// Time zones, and the night the clocks change
convertUtcToZoned("2024-06-15T12:00:00Z", "Asia/Tokyo"); // "2024-06-15T21:00:00+09:00[Asia/Tokyo]"
addZoned("2024-03-10T01:30:00-05:00[America/New_York]", { hours: 1 }); // "2024-03-10T03:30:00-04:00[America/New_York]"
resolveLocal("2024-03-10T02:30:00", "America/New_York", { disambiguation: "reject" }); // "" (02:30 did not happen that day)

Every function has a reference page with a live playground: plain · zoned · utc · unix · instant · calendar · duration · interval · span · precision

By industry: transport · intermodal

Then the guides show how to do each job, and the tools let you try it on your own values.

Agent prompt

Paste this into your AI coding agent:

You are a coding assistant using @northguild/gmt — a Temporal-first date/time library.

SETUP (do this once per project):

1. Install the runtime:
   npm install @northguild/gmt

2. (Optional) Install a linter plugin for Date-ban enforcement:
   npm install -D @northguild/gmt-eslint   # ESLint
   npm install -D @northguild/gmt-oxlint   # Oxlint
   npm install -D @northguild/gmt-biome    # Biome

3. Wire TanStack Intent so skill guidance is discoverable in AGENTS.md:
   npx @tanstack/intent@latest install

WHEN HELPING THE USER:

1. Ask what difficulties they are having with JavaScript dates — this helps match
   them to the right task area (basics, arithmetic, timezone, integration).

2. Generate code using GMT's string-in/string-out API. NEVER use new Date().
   For API details, read https://gmt-dox.northguild.workers.dev/llms.txt and the
   JSDoc in the installed package.

3. For specialized tasks, use TanStack Intent to discover and load the relevant skill:
   npx @tanstack/intent@latest list
   npx @tanstack/intent@latest load @northguild/gmt#<skill-name>

License

MIT. See LICENSE.