timezone_finder

An add-on for package:timezone. It turns a point on Earth into a Location, adds English time zone names, and converts a TZDateTime from one place to another.

Initialization

timezone_finder is deeply connected to the official timezone package, so two datasets have to be ready first:

1. tzdata — IANA zone rules, from package:timezone. Always load latest_all. latest drops the tzdb link identifiers — 106 of this package's 419 boundary identifiers are missing from it, and a lookup that hits one throws.
2. Boundaries — land polygons, from this package. Already compiled in on the VM. On web they are a ~4 MB .bin you install once at startup.

On web, import package:timezone_finder/browser.dart instead of package:timezone_finder/timezone_finder.dart. It is web-only and re-exports the same API. A Flutter app that also targets mobile or desktop should conditionally import browser.dart on the same branch that loads the .bin — importing it off web is a compile error; using timezone_finder.dart on web compiles, then throws at lookup.

VM, CLI, server, Flutter mobile and desktop

• Import
  • package:timezone/data/latest_all.dart and package:timezone/timezone.dart
  • package:timezone_finder/timezone_finder.dart
• Start the code with
  • tz.initializeTimeZones() to load tzdata for the timezone package.

import 'package:timezone/data/latest_all.dart' as tz;
import 'package:timezone/timezone.dart';
import 'package:timezone_finder/timezone_finder.dart';

void main() {
  tz.initializeTimeZones();

  final paris = findLocation(2.3522, 48.8566); // Europe/Paris Location
}

On a server, await ensurePreloaded() at isolate startup if you do not want the first request to pay the decode.

Dart web

• Import
  • package:timezone/browser.dart
  • package:timezone_finder/browser.dart
• Start the code with
  • await tz.initializeTimeZone('packages/timezone/data/latest_all.tzf'), not latest.tzf (default)
  • await initializeBoundaries() fetches the .bin from packages/timezone_finder/data/boundaries_2026c.bin without any asset pipeline. Pass a URL to self-host.

import 'package:timezone/browser.dart' as tz;
import 'package:timezone_finder/browser.dart';

Future<void> main() async {
  await tz.initializeTimeZone('packages/timezone/data/latest_all.tzf');
  await initializeBoundaries();

  final paris = findLocation(2.3522, 48.8566); // Europe/Paris Location
}

Flutter web

• Declare the .bin in your app's pubspec.yaml:

flutter:
  assets:
    - packages/timezone_finder/data/boundaries_2026c.bin

The timezone_finder package does not declare the .bin as a Flutter asset, so Android and iOS builds — which use the embedded index — do not carry 4 MB they never read.

• Import
  • package:timezone/browser.dart
  • package:timezone_finder/browser.dart
• Start the code with
  • await tz.initializeTimeZone('packages/timezone/data/latest_all.tzf'), not latest.tzf (default)
  • installBoundaries(...) from rootBundle

import 'package:flutter/services.dart';
import 'package:flutter/widgets.dart';
import 'package:timezone/browser.dart' as tz;
import 'package:timezone_finder/browser.dart';

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();

  await tz.initializeTimeZone('packages/timezone/data/latest_all.tzf');

  final data = await rootBundle.load(
    'packages/timezone_finder/data/boundaries_2026c.bin',
  );
  installBoundaries(data.buffer.asUint8List());

  final paris = findLocation(2.3522, 48.8566); // Europe/Paris Location

  runApp(const MyApp());
}

findLocation, ensurePreloaded, and boundaryDataVersion throw a StateError until the .bin is installed. Installing the same data version again still decodes the bytes but does not replace the index; a different version replaces it. initializeBoundaries fetch failures throw BoundariesInitException; corrupt bytes throw IndexFormatException.

API

After initialization, the surface is small. Argument order is GeoJSON: longitude, then latitude.

🎯 Find a Location

null means no land polygon covers the point. Coastal belts extend ~22 km offshore, so a narrow strait often resolves and open ocean does not. Bad coordinates throw ArgumentError; malformed GeoJSON throws FormatException. Missing tzdata, latest instead of latest_all, or a web lookup before install throw StateError. null is never an error.

final home = findLocation(-118.6884215, 34.0327862)!; // America/Los_Angeles Location

final hotelJson =
    '{"type": "Point", "coordinates": [151.2430457, -33.8760146]}';
final hotel = hotelJson.findLocation()!; // Australia/Sydney Location

Pass one Feature or a bare Point, not a geocoder's whole FeatureCollection. Nominatim and Photon both return a collection; pick the feature you want, then call findLocation on that feature's JSON.

🌐 Get time zone names

A Location is a place. A TZDateTime is a place and a moment.

• Location.timeZoneGenericName — CLDR generic, or standard when the zone is not seasonal (Pacific Time, Azerbaijan Time). null for nine zones CLDR leaves unnamed.
• TZDateTime.timeZoneLongName — name at this instant (Pacific Daylight Time, Central European Summer Time). null in a few cases; timeZoneLongName ?? utcOffsetLabel is never null.
• TZDateTime.utcOffsetLabel — UTC-07 / UTC+04:30 / UTC.

TZDateTime.timeZoneName is already on package:timezone (tzdb abbreviation: PDT, or a numeric form like -03).

print('${home.name} → ${home.timeZoneGenericName}');
// America/Los_Angeles → Pacific Time

final call = TZDateTime(home, 2026, 8, 14, 18, 30);

print(call);                  // 2026-08-14 18:30:00.000-0700
print(call.timeZoneLongName); // Pacific Daylight Time
print(call.timeZoneName);     // PDT
print(call.utcOffsetLabel);   // UTC-07

🕘🕔 Convert the same instant to another clock

toLocation is the Location analogue of toUtc / toLocal. offsetDifference is the signed gap between the two offsets at that instant — not travel time, and not TZDateTime.difference (elapsed time). Use .abs() for a directionless magnitude.

final callInSydney = call.toLocation(hotel);

print(callInSydney);                  // 2026-08-15 11:30:00.000+1000
print(callInSydney.timeZoneLongName); // Australian Eastern Standard Time
print(callInSydney.timeZoneName);     // AEST
print(callInSydney.utcOffsetLabel);   // UTC+10

print(call.offsetDifference(hotel));  // 17:00:00.000000 — Sydney is ahead

Shipped data versions: boundaryDataVersion (Timezone Boundary Builder release) and cldrVersion. They are independent of the tzdb version inside package:timezone.

Data used

Dataset	Source	What it answers
Land polygons	Timezone Boundary Builder from OpenStreetMap	Which IANA zone contains this longitude/latitude
English names	Unicode CLDR	Pacific Time, Central European Summer Time, …
Zone rules	package:timezone (IANA tzdb)	Offsets, DST, abbreviations

Coordinates come from wherever you already have them — a GPS fix, a map tap, a database column. From an address, Nominatim and Photon return GeoJSON that String.findLocation reads directly.

Licenses

• Code — MIT (LICENSE).
• Boundary data — ODbL v1.0 (LICENSE-DATA). Share-alike: if you publicly ship an adapted database, offer that adapted database under ODbL too.
• CLDR English names — Unicode License v3 (LICENSE-CLDR).

If you redistribute the boundary data (or a work produced from it), include:

Time zone boundary data © OpenStreetMap contributors, available under the Open Database License (ODbL). Boundaries built by Timezone Boundary Builder.

IANA publishes no official polygons; tz-link cites TZBB as the de facto source. TZBB is by Evan Siroky, built from OpenStreetMap data.