format_date_with_offset_test.dart

import 'package:flutter_test/flutter_test.dart';
import 'package:intl/intl.dart';
import 'package:timezone/timezone.dart';
import 'package:timezone/data/latest.dart';

// A DateTime in dart contains the TimezoneOffset from UTC/GMT
// However formatting a Date using timezone patterns/symbols like "Z" or "Z"
// is not supported yet.
// See: https://github.com/dart-lang/intl/issues/19 for more details.
Future<void> main() async {
  initializeTimeZones();

  test('format date that includes offset', () {
    var date = TZDateTime.now(getLocation('Asia/Taipei'));
    var formattedDate = formatDateWithOffset(date);

    expect(formattedDate, contains("+08:00"));
  });

  test('double digit offset hours', () {
    var brisbane = getLocation('Australia/Brisbane');
    var date = TZDateTime.now(brisbane);
    var formattedDate = formatDateWithOffset(date);

    expect(formattedDate, contains("+10:00"));
  });

  test('single digit offset hours', () {
    final chicago = getLocation('America/Chicago');
    var date = TZDateTime.now(chicago);
    var formattedDate = formatDateWithOffset(date);

    expect(formattedDate, contains("-05:00"));
  });

  test('30 minute offset', () {
    final brokenHill = getLocation('Australia/Broken_Hill');
    var date = TZDateTime.now(brokenHill);
    var formattedDate = formatDateWithOffset(date);

    expect(formattedDate, contains(":30"));
  });

  test('across several different timezones', () {
    final chicago = getLocation('America/Chicago');
    var date = TZDateTime.now(chicago);
    var formattedDate = formatDateWithOffset(date);
    expect(formattedDate, contains("-05:00"));

    final detroit = getLocation('America/Detroit');
    date = TZDateTime.now(detroit);
    formattedDate = formatDateWithOffset(date);
    expect(formattedDate, contains("-04:00"));

    // Phoenix
    final phoenix = getLocation('America/Phoenix');
    date = TZDateTime.now(phoenix);
    formattedDate = formatDateWithOffset(date);
    expect(formattedDate, contains("-07:00"));

    // Denver
    final denver = getLocation('America/Denver');
    date = TZDateTime.now(denver);
    formattedDate = formatDateWithOffset(date);
    expect(formattedDate, contains("-06:00"));

    final la = getLocation('America/Los_Angeles');
    date = TZDateTime.now(la);
    formattedDate = formatDateWithOffset(date);
    expect(formattedDate, contains("-07:00"));

    final kolkata = getLocation('Asia/Kolkata');
    date = TZDateTime.now(kolkata);
    formattedDate = formatDateWithOffset(date);
    expect(formattedDate, contains("+05:30"));

    final brisbane = getLocation('Australia/Brisbane');
    date = TZDateTime.now(brisbane);
    formattedDate = formatDateWithOffset(date);
    print(formattedDate);
    expect(formattedDate, contains("+10:00"));

    final honolulu = getLocation('Pacific/Honolulu');
    date = TZDateTime.now(honolulu);
    formattedDate = formatDateWithOffset(date);
    expect(formattedDate, contains("-10:00"));

    final zurich = getLocation('Europe/Zurich');
    date = TZDateTime.now(zurich);
    formattedDate = formatDateWithOffset(date);
    expect(formattedDate, contains("+02:00"));

    final london = getLocation('Europe/London');
    date = TZDateTime.now(london);
    formattedDate = formatDateWithOffset(date);
    expect(formattedDate, contains("+01:00"));
  });
}

/// Formats Date and includes the time zone offset in iso8601 format
String formatDateWithOffset(DateTime date) {
  String twoDigits(int n) => n >= 10 ? "$n" : "0$n";

  var hours = twoDigits(date.timeZoneOffset.inHours.abs());
  var minutes = twoDigits(date.timeZoneOffset.inMinutes.remainder(60));
  var sign = date.timeZoneOffset.inHours > 0 ? "+" : "-";
  var formattedDate = DateFormat("yyyy-MM-ddTHH:mm:ss").format(date);

  return "$formattedDate$sign$hours:$minutes";
}