Timezones for ZeppOS

Timezones - A small library that properly handles Daylight Saving Time (DST) changes.

• Handles timezone conversions, DST calculations, and location-based time information.
• Supports IANA timezone identifiers, UTC offsets, and provides methods for getting current time, converting between timezones, and determining DST status.
• Uses a built-in timezone database that is suited to work fully offline.
• Allows simple modification & simulation of actual time and date.

Installation

To install the library, run the following command from the root of your project

npm i @silver-zepp/timezones

Download example (optional)

The library contains an example suited to run on Balance watch/emulator.

• modify app.json if you want to run this example on other watches

To use the library, clone this repo and run these commands from the root of the /examples/timezones-example folder.

npm install
zeus dev

Using the library

// install -> npm i @silver-zepp/timezones
import { Timezones } from "@silver-zepp/timezones";

// sets your tz instance to UTC -4 or -5 depending on Daylight Saving Time (DST)
const tz = new Timezones("America/New_York");
// by country abbriviation
const tz = new Timezones("CA"); // Canada
// or keep the built-in timezone but still be able to use benefits of the library
const tz = new Timezones();

// create a valid Date object that is shifted towards your specific timezone
// equivalent to const date = new Date();
const date = tz.getDate();
const cur_hour = date.getHours();
const cur_ms = date.getMilliseconds();
// ...
console.log(date);      // Tue Nov 26 2024 23:26:24 GMT-05:00 (EST)
console.log(cur_hour);  // 0-23

// alternatively you can get current HH/MM/SS without explicit date object creation
const cur_hour = tz.getSeconds(); // 0-59

// get current location and DST status
const { location, is_dst } = tz.getLocationAndDaylightStatus();
console.log(location, is_dst); // output: "America/New_York" false

Bonus methods

Print approximate location based on GPS coordinates

const london = { lat: 51.5072, lon: 0.1276 };
let approximate_location = tz.getApproxLocation(london.lat, london.lon);
console.log("GPS Approximate location:", approximate_location);
// output: GPS Approximate location: Europe/London

Use cases

• can be used in combination with built-in watch's GPS sensor to create a basic Geolocator
  • allows wearers to approximate their position, such as identifying the city they're flying over even in Airplane mode (no internet or Wi-Fi)
• can serve as an emergency location estimator in extreme scenarios like remote crash landings (as depicted in the 2023 film "Society of the Snow")
  • while rare, this capability could be crucial in isolated or "dead-zone" areas

Print when will the next DST change happen

// return a next dst change object that represents the time until the next DST change
console.log(JSON.stringify(tz.getTimeUntilNextDstChange()));
// output: 
{
  "next_change":"2025-03-02T02:00:00.000Z",
  "time_until_change":8213891000,
  "change_to_dst":true
}

// pretty print the date for the next DST on/off change
console.log(tz.formatTimeUntilNextDstChange());
// output: 
Time until DST starts: 95 days, 1 hours, 38 minutes, and 11 seconds 

Use cases

• can be used to create a simple app that counts time until Summer/Winter or a reminder to adjust the clocks that don't have internet

Set a Simulated Date

• Sets a simulated current date for testing purposes or time travel :)

// 4H 1M before NY DST OFF (doesn't accoutn for current TZ shift)
Timezones.SetCurrentDate(new Date('2024-11-03T01:59:00Z')); 

Convert Local Time to Other Timezone Time

// convert current (NY) time to Tokyo time
console.log("Time in Japan:", tz.convertToTimeZone(tz_date, 'Asia/Tokyo'));
// output: 
Time in Japan: Wed Nov 27 2024 14:21:48 // +14 hours to NY

Print related info regardless of a particular timezone

const tz_info = tz.getTimezoneInfo("US");
vis.log(JSON.stringify(tz_info));
// output: 
{
  "code":"US",
  "tz_id":"America/New_York",
  "utc_sdt":"-05:00",
  "utc_dst":"-04:00",
  "tz_sdt":"EST",
  "tz_dst":"EDT",
  "continent":"northamerica",
  "lat":40.7127,
  "lon":-74.006,
  "dst_rule":"B13"
} 

DST rule (dst_rule) HEX encoding explained

• Handles both northern and southern hemisphere DST rules.
• Considers DST changes occurring at 2 AM.
• Handles the "last" week of the month when n is 0 in nthWeekdayOfMonth.
• Example for NY "dst_rule: B13" -> B13 (hex) = 0000 1011 0001 0011 (bin, 16 bits) start month: 3 (March) start week: 1 (First Sunday) end month: 11 (November) end week: 0

+-------+-------+-------+-------+-------+
| Bits  | 15-12 | 11-8  |  7-4  |  3-0  |
+-------+-------+-------+-------+-------+
| Desc  | End   | End   | Start | Start |
|       | Week  | Month | Week  | Month |
+-------+-------+-------+-------+-------+
| Value |  0000 | 1011  | 0001  | 0011  |
|       |  (0)  | (11)  |  (1)  |  (3)  |
+-------+-------+-------+-------+-------+

More about the library

This library works with ZeppOS V2 and up!

Supported Constructor Formats

// empty constructor = use hardware/emulator built-in TZ
const tz = new Timezones(); 
// hour shifts in different formats
const tz = new Timezones(-4);       // int
const tz = new Timezones("-5");     // str int
const tz = new Timezones("+04:30"); // str
// valid / slightly misspelled IANA format
// WrongCountry/WrongCity | Europe/London | Europe/WarZaw
const tz = new Timezones("America/New_York");
// (str) "CA" // country abbriviation for Canada
const tz = new Timezones("CA");
// (str) SDT | DST (lookup)
const tz = new Timezones("CST");

Cache clearing

If the location changes quickly or something doesn't look right, try changing the cache.

tz.clearCache();

Other methods

const is_dst = tz.getDaylightStatus('America/New_York');
console.log(is_dst); 
// output: true (during DST period)

const { location, is_dst } = tz.getLocationAndDaylightStatus();
console.log(location, is_dst); 
// output: "America/New_York" true

const cur_time = tz.getTime();
console.log(cur_time); 
// output: ISO string -> "2024-10-10T14:30:00.000-04:00"