glintl

Format dates and times in many languages, on Erlang and JavaScript.

glintl isn’t ready to depend on yet. Expect breaking changes.

On Erlang it aims to have the same output as JavaScript’s Intl.DateTimeFormat.

gleam add glintl gleam_time

import gleam/time/duration
import gleam/time/timestamp
import glintl
import glintl/locale/en_gb
import glintl/locale/fr

pub fn main() {
  let now = timestamp.system_time()
  let offset = duration.hours(1)

  glintl.date_time_formatter(en_gb.locale(), date: glintl.Long, time: glintl.Short)
  |> glintl.format(now, offset:)
  // -> "25 October 2026 at 23:40"

  glintl.date_formatter(fr.locale(), style: glintl.Full)
  |> glintl.format(now, offset:)
  // -> "dimanche 25 octobre 2026"
}

It is recommended to reuse formatters for performance, because making one is expensive and does lookup work.

Styles

Locales

• Prefer importing specific locales if you can: glintl/locale/en_gb.

• You can build a subset of desired locales by importing many.

• Otherwise use glintl/locales and locales.from_tag("en-GB") if you need them all, or only know at runtime.

  import glintl/locales
  
  locales.from_tag("en-GB") // -> Ok(en_gb.locale())
  locales.from_tag("en-XX") // -> Ok(en.locale())
  locales.from_tag("xx") // -> Error(Nil)
  

Note that importing glintl/locales will load every locale’s data, which can be wasteful if you do not need all the locales in your application. Always prefer importing specific locales if you can.

Calendars

• Each locale uses its own calendar by default. Thai uses Buddhist.
• Use other calendars with glintl.with_calendar(locale, glintl.Hebrew).
• All of ICU’s calendars: Gregorian, ISO 8601, Buddhist, Japanese, Republic of China, Persian, Indian, Hebrew, Chinese, Dangi, Coptic, Ethiopic (2 eras), and 3 Islamic calendars.

How it works

On Erlang:

• Formats come from ICU when glintl is built, not when your app runs.
• Each locale data is defined as a literal in its own module. The result is improved memory usage, because the data can be safely shared by every process and it is rarely copied or garbage collected.
• We’re working on making the start-up cost smaller.

On JavaScript:

• glintl calls Intl.DateTimeFormat.
• A locale is just its tag, so your bundle has no locale data.

Limits

• Today, time zones in Glintl are passed as offsets. You get GMT+1, not British Summer Time.
• Today, Glintl does not allow you to pick single fields like month and year only.
• On Erlang, calendars with their own months cover the years 1 to 4000. Other dates fall back to Gregorian.

Known differences

• Erlang vs JavaScript: Because the ICU data bundled in Glintl (for Erlang), and the ICU in your JavaScript runtime may differ, output can change between platforms. This usually manifests in small details like spaces and commas.
• ICU bugs glintl copies: ICU 78.3 puts Chinese New Year 2027 and 2030 a day off. JavaScript does the same.
• ICU bug glintl doesn’t copy yet: near AD 284 and AD 622, the Coptic and Islamic calendars can show the wrong year around midnight.

Contributing

bun install --cwd codegen
bun fmt --cwd codegen && bun lint --cwd codegen && bun check --cwd codegen
gleam test --target erlang
gleam test --target javascript
gleam run -m glintl_bench
bun codegen/conformance.ts 1000000

• The generator, JavaScript tests and conformance run need the Bun in .bun-version, on Linux. CI does this.
• On a Mac, run the generator in Docker. It takes about 10 minutes:

docker run --rm -v "$PWD":/repo -w /repo oven/bun:$(cat .bun-version) bun codegen/generate.ts
gleam format src test