$ snippet timezone-dst-safe-dates

Convert dates across time zones (DST-safe)

Format any instant in a named IANA time zone, and get the current UTC offset for that zone — daylight saving handled by the browser.

JavaScript

/** Format an instant in a given IANA time zone, e.g. "America/New_York". */
function formatInZone(date, timeZone, locale = "en-US") {
  return new Intl.DateTimeFormat(locale, {
    timeZone, year: "numeric", month: "2-digit", day: "2-digit",
    hour: "2-digit", minute: "2-digit", second: "2-digit", hour12: false,
    timeZoneName: "short",
  }).format(date);
}

/** UTC offset (in minutes) of a zone at a given instant. Accounts for DST. */
function zoneOffsetMinutes(date, timeZone) {
  const parts = new Intl.DateTimeFormat("en-US", {
    timeZone, hourCycle: "h23", year: "numeric", month: "2-digit", day: "2-digit",
    hour: "2-digit", minute: "2-digit", second: "2-digit",
  }).formatToParts(date).reduce((o, p) => (o[p.type] = p.value, o), {});
  const asUTC = Date.UTC(parts.year, parts.month - 1, parts.day, parts.hour, parts.minute, parts.second);
  return Math.round((asUTC - date.getTime()) / 60000);
}

/** Build a Date from wall-clock components in a zone ("9:00 in Tokyo"). */
function dateFromZone(y, m, d, h, min, timeZone) {
  const guess = new Date(Date.UTC(y, m - 1, d, h, min));
  const offset = zoneOffsetMinutes(guess, timeZone);
  const result = new Date(guess.getTime() - offset * 60000);
  // second pass in case the guess straddled a DST switch
  const offset2 = zoneOffsetMinutes(result, timeZone);
  return offset2 === offset ? result : new Date(guess.getTime() - offset2 * 60000);
}

How it works

A JavaScript Date is just a UTC timestamp; the time zone only matters when you format it or build it from parts. Intl.DateTimeFormat knows every zone's DST rules, so you never hard-code offsets like -5. The third helper solves the common "user entered 9:00 in their zone, store it as UTC" problem, including the day the clocks change.

Gotchas

Never store or compare a fixed offset like UTC-5 — New York is -5 in winter and -4 in summer. Store the IANA zone name and a UTC instant.
Adding 24 hours is not the same as adding one day across a DST change (that day is 23 or 25 hours long). Add days via wall-clock parts, not milliseconds.
date.getTimezoneOffset() gives the browser's offset, not the user's chosen zone.
The Temporal API (Temporal.ZonedDateTime) replaces all of this once it lands in all browsers.

Try it

Result

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