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    1.3.2 • Public • Published


    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.

    // 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"


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

    Compatible with Browsers, and Node.js.


    <script src=""></script>
    var TZ = window.XTZ;

    Node.js & Webpack

    npm install --save xtz
    var TZ = require("xtz");



    How was this built?

    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:

    (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)

    toTimeZone(utcDate, timeZone)

    Convert UTC into a Target Time Zone

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

    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"
    // }
    // "2021-03-14T03:15:59.000-0400"
    // (same as "2021-11-07T07:15:59.000Z")

    Convert directly to an ISO String:

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

    Or use our bespoke (custom) date object:

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

    You can also use a date object with an absolute ISO time:

    var tzDate = TZ.toTimeZone(
      new Date("2021-11-07T08:15:59.000Z"),
    // "2021-11-07T03:15:59.000-0500"

    Our ISO Strings + Offsets work with JavaScript's native Date object!!

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

    fromTimeZone(dtString, timeZone)

    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"
    // }
    // "2021-11-07T03:15:59.000-0500"
    // (same as "2021-11-07T08:15:59.000Z")

    Convert directly to an offset ISO String:

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

    Or our bespoke date object:

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

    Use a Date as a source time

    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"),
    // "2021-11-07T03:15:59.000-0500"

    Daylight Savings / Edge Cases

    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)


    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");
      // "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");
      // "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")

    List of Time Zones

    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)


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