GMT: Give Me Temporal

The most tested date and time library for JavaScript and TypeScript. @northguild/gmt handles time zones, DST and business days in 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.

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import {
  addDate,
  diffDate,
  endOfDate,
  formatDate,
  isBeforeDate,
  startOfDate,
  subtractDate,
} from "@northguild/gmt";

addDate("2024-01-31", { months: 1 }); // "2024-02-29"
subtractDate("2024-03-31", { days: 7 }); // "2024-03-24"
diffDate("2024-01-01", "2024-03-15", "days"); // 74
startOfDate("2024-03-15", "month"); // "2024-03-01"
endOfDate("2024-03-15", "month"); // "2024-03-31"
isBeforeDate("2024-03-15", "2024-04-01"); // true
formatDate("2024-03-15", "en-US", { dateStyle: "long" }); // "March 15, 2024"

Date in addDate names the type, not a catch-all. Each kind of time has its own set of functions: a date, a time of day, a date and time, a zoned time, a UTC instant and a Unix timestamp. Each takes and returns only its own kind, so what you pass in tells you what you get back.

import { addDate, addDateTime, addTime, addUnix, addUtc, addZoned } from "@northguild/gmt";

addDate("2024-03-15", { days: 30 }); // "2024-04-14"
addTime("22:30:00", { hours: 3 }); // "01:30:00"
addDateTime("2024-03-15T22:30:00", { hours: 3 }); // "2024-03-16T01:30:00"
addZoned("2024-03-15T22:30:00+01:00[Europe/Berlin]", { hours: 3 }); // "2024-03-16T01:30:00+01:00[Europe/Berlin]"
addUtc("2024-03-15T22:30:00Z", { hours: 3 }); // "2024-03-16T01:30:00Z"
addUnix(1710541800000, { hours: 3 }); // 1710552600000

The same pattern holds for every operation: diff, startOf, endOf, format, isBefore and the rest, for each type.

Time zones and daylight saving time stay just as direct:

import { addZoned, resolveLocal } from "@northguild/gmt";

// Berlin's clocks jump from 02:00 to 03:00 on 31 March 2024, so a day and 24 hours are not the same.
addZoned("2024-03-30T12:00:00+01:00[Europe/Berlin]", { days: 1 }); // "2024-03-31T12:00:00+02:00[Europe/Berlin]"
addZoned("2024-03-30T12:00:00+01:00[Europe/Berlin]", { hours: 24 }); // "2024-03-31T13:00:00+02:00[Europe/Berlin]"
// 02:30 that night never happened. You choose what to get, and "reject" returns "".
resolveLocal("2024-03-31T02:30:00", "Europe/Berlin", { disambiguation: "reject" }); // ""

• 735 functions, to the nanosecond, tested in 17 locales.
• Never throws. ISO 8601 strings in. Invalid input returns "", null, false or [].

The latest test counts, CI volume and the comparison with Luxon, date-fns, Moment.js, Day.js, Spacetime and @internationalized/date are on the Why GMT page. The full comparison and the method are on the compare page.

Moving from another library: date-fns · Luxon · Day.js · Moment.js · @internationalized/date.

Install

npm install @northguild/gmt

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

Use

import { addBusinessDays, convertUtcToZoned, isBusinessDay } from "@northguild/gmt";

addBusinessDays("2024-03-15", 1); // "2024-03-18"
isBusinessDay("2024-03-16"); // false
convertUtcToZoned("2024-06-15T12:00:00Z", "Asia/Tokyo"); // "2024-06-15T21:00:00+09:00[Asia/Tokyo]"

Every function has a reference page with a live playground: plain · zoned · utc · unix · instant · calendar · duration · interval · workforce · span · precision · recurrence · timescale · gnss · transport · intermodal

By industry: Workforce · Transport · Intermodal · GNSS · Maritime

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://northguild.dev/gmt/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. The four npm packages in packages/ and the root LICENSE are MIT.

Every other folder is © 2026 NorthGuild, all rights reserved, under the LICENSE file in that folder: apps/ (the docs site), context/, .agents/, .claude/, scripts/ and docs/. The root files AGENTS.md and CLAUDE.md are reserved too, under .agents/LICENSE. Code examples in the documentation may be used under MIT-0 (MIT No Attribution); see apps/LICENSE.