glintl

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

Glintl lazy-loads locales for performance. 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

• Import the ones you need: import glintl/locale/ja.

• Pick one from a tag with locales.from_tag("en-GB").

  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)
  

• glintl/locales imports every locale. Only use it if you don’t know the locale until run time.

Calendars

• Each locale uses its own calendar by default. Thai uses Buddhist.
• Pick another 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 is an Erlang module. It loads the first time you use it, and stays loaded.
• The data is shared by every process. It is never copied or garbage collected.
• Releases in “embedded mode” load every module at start-up instead.

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: Erlang uses the ICU in Bun on Linux. JavaScript uses your runtime’s ICU. Differences can exist, usually manifesting 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