Get utc offset from timezone in Javascript

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It has become possible nowaday with Intl API:

The implementation of Intl is based on icu4c. If you dig the source code, you’ll find that timezone name differs per locale, for example:

for (const locale of ["ja", "en", "fr"]) {
  const timeZoneName = Intl.DateTimeFormat(locale, {
    timeZoneName: "short",
    timeZone: "Asia/Tokyo",
  })
    .formatToParts()
    .find((i) => i.type === "timeZoneName").value;
  console.log(timeZoneName);
}

Fortunately, there is a locale, Interlingua (the langauge tag is ia), which uses the same pattern (ex. GMT+11:00) for timezone names.

The snippet below can meed your need:

const getOffset = (timeZone) => {
  const timeZoneName = Intl.DateTimeFormat("ia", {
    timeZoneName: "short",
    timeZone,
  })
    .formatToParts()
    .find((i) => i.type === "timeZoneName").value;
  const offset = timeZoneName.slice(3);
  if (!offset) return 0;

  const matchData = offset.match(/([+-])(\d+)(?::(\d+))?/);
  if (!matchData) throw `cannot parse timezone name: ${timeZoneName}`;

  const [, sign, hour, minute] = matchData;
  let result = parseInt(hour) * 60;
  if (sign === "+") result *= -1;
  if (minute) result += parseInt(minute);

  return result;
};

console.log(getOffset("US/Eastern")); // 240
console.log(getOffset("Atlantic/Reykjavik")); // 0
console.log(getOffset("Asia/Tokyo")); // -540

This way can be a little tricky but it works well in my production project. I hope it helps you too 🙂

Update

Many thanks to Bort for pointing out the typo. I have corrected the snippet.

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