Date time parse and format as ISO-8601 with local timezone

main.dart

/*
 * Date time parse and format as ISO-8601 with local timezone
 * https://gist.github.com/PlugFox/339bcd8dbec29a56fd53f8e0656e4a54
 * https://dartpad.dev/339bcd8dbec29a56fd53f8e0656e4a54?id=&null_safety=true
 */

import 'dart:async';
import 'package:intl/intl.dart' as intl;

const String source = '2021-01-02T03:08:04+02:30';

void main() => runZonedGuarded<void>(
      () async {
        final dateTime = DateTime.tryParse(source);
        if (dateTime == null) throw FormatException('Invalid date format');
        print(dateTime.toLocalIso8601String());
      },
      (error, stackTrace) => print('ERROR: $error'),
    );

extension LocalTimeX on DateTime {
  static final intl.DateFormat _formatter =
      intl.DateFormat("yyyy-MM-dd'T'HH:mm:ss");

  String toLocalIso8601String() {
    final dateTime = toLocal();
    final tz = dateTime.timeZoneOffset;

    final buffer = StringBuffer()
      ..write(_formatter.format(dateTime))
      ..write(tz.isNegative ? '-' : '+')
      ..write(tz.inHours.abs().toString().padLeft(2, '0'))
      ..write(':')
      ..write((tz.inMinutes.abs() % 60).toString().padLeft(2, '0'));

    return buffer.toString();
  }
}