xtz.js

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:

``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"
`

`js
// 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"
`

`js
// 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"
`

Features

- [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.

Browsers

`html

`

`js
var TZ = window.XTZ;
`

Node.js & Webpack

`bash
npm install --save xtz
`

`js
var TZ = require("xtz");
`

Demo

See .

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:
https://www.youtube.com/playlist?list=PLxki0D-ilnqa6horOJ2G18WMZlJeQFlAt

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

API

- 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):

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

`js
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")
`

$3

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

$3

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

$3

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

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

$3

`js
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:

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

`js
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")
`

$3

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

$3

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

$3

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).

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

`js
utcDate.toISOString();
// "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)
>
> See .

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.
`js
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.
`js
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")
`

List of Time Zones

See the Full List of Time Zones on Wikipedia.

Common Zones for Testing:

`txt
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)
``