A fast, lightweight, zero-dependency library to translate between Time Zones and UTC with native Intl.DateTimeFormat in ~100 LoC. For Node.js & Browsers.

XTZ is a poor man's Temporal polyfill, but just for time zones.
Demo: https://therootcompany.github.io/tz.js/

// What's the current time, in ISO+Offset format?

TZ.toLocalISOString(new Date()); // "2021-11-07T03:15:59.000-0500"
TZ.timeZone(); // "America/New_York"

// What will the ISO+Offset datetime string be
// when it's 3:15am in New York?
//
// (Relative New York time to Absolute ISO+Offset Time)

TZ.toOffsetISOString("2021-11-07 03:15:59.000", "America/New_York");
// "2021-11-07T03:15:59.000-0500"

// What time will it be in New York
// when it's 7:15am UTC?
//
// (Absolute UTC Zulu time to Relative New York time)

TZ.toTimeZoneISOString("2021-03-14T07:15:59.000Z", "America/New_York");
// "2021-03-14T03:15:59.000-0400"

• Translate a UTC Zulu time to a Time Zone
• Translate a Zoned time to ISO+Offset
• Handles Daylight Savings, Weird Time Zones, etc...
  • Well-tested npm run test
• Lightweight (No deps)
  • 5kb Source + Comments
  • 2.5kb Minified
  • <1kb gzipd

Compatible with Browsers, and Node.js.

<script src="https://unpkg.com/xtz@latest/xtz.min.js"></script>

var TZ = window.XTZ;

npm install --save xtz

var TZ = require("xtz");

See https://therootcompany.github.io/tz.js/.

I live-streamed the creation of this entire project.

If you'd like to learn how I did it and what challenges I encountered, you can watch here: https://www.youtube.com/playlist?list=PLxki0D-ilnqa6horOJ2G18WMZlJeQFlAt

(though there have been a few minor updates and bug fixes off-camera)

• toLocalISOString(dateOrNull)
• toTimeZone(utcDate, timeZone)
• toTimeZoneISOString(isoString, timeZone)
• fromTimeZone(dtString, timeZone)
• toOffsetISOString(dtString, timeZone)

Convert UTC into a Target Time Zone

Use ISO timestamps representing the absolute UTC time (ISO with or without offset):

"2021-11-07T08:15:59.000Z"

var utcDate = TZ.toTimeZone("2021-03-14T07:15:59.000Z", "America/New_York");
// {
//   year: 2021, month: 2, day: 14,
//   hour: 3, minute: 15, second: 59, millisecond: 0,
//   offset: -240, timeZoneName: "Eastern Daylight Time"
// }

utcDate.toISOString();
// "2021-03-14T03:15:59.000-0400"
// (same as "2021-11-07T07:15:59.000Z")

TZ.toTimeZoneISOString("2021-11-07T08:15:59.000Z", "America/New_York");
// "2021-11-07T03:15:59.000-0500"

var tzDate = TZ.toTimeZone("2021-11-07T08:15:59.000Z", "America/New_York");

var tzDate = TZ.toTimeZone(
  new Date("2021-11-07T08:15:59.000Z"),
  "America/New_York"
);

console.log(tzDate.toISOString());
// "2021-11-07T03:15:59.000-0500"

new Date("2021-11-07T03:15:59.000-0500").toISOString());
// "2021-11-07T08:15:59.000Z"

Convert a Target Time Zone into ISO

Use ISO-like timestamps representing the local time in the target time zone:

"2021-11-0 03:15:59.000"

var tzDate = TZ.fromTimeZone("2021-11-07 03:15:59.000", "America/New_York");
// {
//   year: 2021, month: 10, day: 7,
//   hour: 3, minute: 15, second: 59, millisecond: 0,
//   offset: -300, timeZoneName: "Eastern Standard Time"
// }

tzDate.toISOString();
// "2021-11-07T03:15:59.000-0500"
// (same as "2021-11-07T08:15:59.000Z")

TZ.toOffsetISOString("2021-11-07 03:15:59.000", "America/New_York");
// "2021-11-07T03:15:59.000-0500"

var utcDate = TZ.fromTimeZone("2021-11-07 03:15:59.000", "America/New_York");

You can also use a date object as the source time, but the date's UTC time will be treated as relative to time zone rather than absolute (this is a workaround for JavaScript's lack of bi-directional timezone support).

var utcDate = TZ.fromTimeZone(
  new Date("2021-11-07T03:15:59.000Z"),
  "America/New_York"
);

utcDate.toISOString();
// "2021-11-07T03:15:59.000-0500"

In 2021 Daylight Savings (in the US)

• begins at 2am on March 14th (skips to 3am)
• ends at 2am on November 7th (resets to 1am)

See https://www.timeanddate.com/time/change/usa.

Q: What happens in March when 2am is skipped?

• A: Although 2am is not a valid time, rather than throwing an error this library will resolve to 1am instead, which is an hour early in real ("tick-tock" or "monotonic") time.

  var utcDate = TZ.fromTimeZone("2021-03-14 02:15:59.000", "America/New_York");
  utcDate.toISOString();
  // "2021-03-14T02:15:59.000-0400"
  // (same as "2021-03-14T01:15:59.000-0500")

Q: What happens in November when 1am happens twice?

• A: Although both 1ams are distinguishable with ISO offset times, only the first can be resolved from a local time with this library.

  var utcDate = TZ.fromTimeZone("2021-11-07 01:15:59.000", "America/New_York");
  utcDate.toISOString();
  // "2021-11-07T01:15:59.000-0400", same as "2021-11-07T05:15:59.000Z"
  // (an hour before the 2nd 1am at "2021-11-07T01:15:59.000-0500")

See the Full List of Time Zones on Wikipedia.

Common Zones for Testing:

America/New_York    -0500
America/Denver      -0700
America/Phoenix     -0700 (No DST)
America/Los_Angeles -0800
UTC                 Z
Australia/Adelaide  +0930 (30-min, has DST)
Asia/Kathmandu      +0545 (No DST, 45-min)
Asia/Kolkata        +0530 (No DST, 30-min)