tz.js/README.md

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# [xtz.js](https://github.com/therootcompany/tz.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.
> What UTC time will it be when it's 3:15am in New York?
```js
// Relative New York time to Absolute UTC Time
TZ.toUTCISOString("2021-11-07 03:15:59.000", "America/New_York");
// "2021-11-07T03:15:59.000-0500"
```
```js
var tzDate = TZ.toUTC("2021-11-07 03:15:59.000", "America/New_York");
// {
// year: 2021, month: 11, 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"
```
> What time will it be in New York when it's 7:15am UTC?
```js
// Absolute UTC time to Relative New York time
TZ.toTimeZoneISOString("2021-11-07T07:15:59.000Z", "America/New_York");
// "2021-11-07T03:15:59.000-0400"
```
```js
var utcDate = TZ.toTimeZone("2021-03-14T07:15:59.000Z", "America/New_York");
// {
// year: 2021, month: 3, 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"
```
# Features
- [x] Translate a UTC time to a Time Zone
- [x] Translate a Zoned time to UTC
- [x] Handles **Daylight Savings**, Weird Time Zones, etc...
- [x] Lightweight (No deps)
- 5kb Source + Comments
- 2.5kb Minified
- <1kb `gzip`d
Compatible with Node.js & Browsers.
## Node.js & Webpack
```bash
npm install --save xtz
```
```js
var TZ = require("xtz");
```
## Browsers
```html
<script src="https://unpkg.com/xtz@1.0.0-rc.1/xtz.min.js"></script>
```
```js
var TZ = window.XTZ;
```
# API
- `toTimeZone(utcDate, timeZone)`
- `toTimeZoneISOString(isoString, timeZone)`
- `toUTC(dtString, timeZone)`
- `toUTCISOString(dtString, timeZone)`
## `toTimeZone(utcDate, timeZone)`
> Convert UTC into a Target Time Zone
Use ISO timestamps representing the absolute UTC time in the target time zone:
```txt
"2021-11-07T08:15:59.000Z"
```
Convert directly to an ISO String:
```js
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:
```js
var tzDate = TZ.toTimeZone("2021-11-07T08:15:59.000Z", "America/New_York");
```
You can also use a date object with an absolute UTC time:
```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"
```
Our ISO Strings + Offsets work with JavaScript's native Date object!!
```js
new Date("2021-11-07T03:15:59.000-0500").toISOString());
// "2021-11-07T08:15:59.000Z"
```
## `toUTC(dtString, timeZone)`
> Convert a Target Time Zone into UTC
Use ISO-like timestamps representing the _local_ time in the target time zone:
```txt
"2021-11-0 T03:15:59.000"
```
Convert directly to an offset ISO String:
```js
TZ.toUTCISOString("2021-11-07 03:15:59.000", "America/New_York");
// "2021-11-07T03:15:59.000-0500"
```
Or our bespoke date object:
```js
var utcDate = TZ.toUTC("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).
```js
var utcDate = TZ.toUTC(
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
> - ends at 2am on November 7th
>
> 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.
```js
var utcDate = TZ.toUTC("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 2am happens twice?
- A: Although both 2ams are distinguishable with ISO offset times, only the first can be resolved from a local time with this library.
```js
var utcDate = TZ.toUTC("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 2am at "2021-11-07T01:15:59.000-0500")
```
# List of Time Zones
See the [Full List of Time Zones](https://en.wikipedia.org/wiki/List_of_tz_database_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
Australia/Adelaide +0930 (30-min, has DST)
Asia/Kathmandu +0545 (No DST, 45-min)
Asia/Kolkata +0530 (No DST, 30-min)
```