RFC3339 DateTime Serializer

date_time_serializer.dart

class DateTimeSerializer {
  static DateTime? deserialize(String? serialized) {
    if (serialized == null) return null;

    final serializedString = serialized;

    final fixed = serializedString.replaceAllMapped(
      RegExp(r'(.*:\d\d)(\.\d+)?(Z|[+-]\d{2}:\d{2})'),
      (match) {
        // #1 capturing group is date and time without a fraction of a second
        // #2 group is `time-secfrac` - if present
        // #3 group is `time-offset`
        var timeSecfrac = match[2] ?? '';

        if (timeSecfrac.length > 7) {
          // 6 is 0th period + (up to) 6 characters accepted by ISO8601
          timeSecfrac = timeSecfrac.substring(0, 7);
        }

        return (match[1] ?? '') + timeSecfrac + (match[3] ?? '');
      },
    );

    return DateTime.parse(fixed).toUtc();
  }

  static String? serialize(DateTime? dateTime) {
    // if (!dateTime.isUtc) {
    //   throw ArgumentError.value(dateTime, 'dateTime', 'Must be in utc for serialization.');
    // }
    if (dateTime == null) return null;
    if (!dateTime.isUtc) {
      final utc = dateTime.toUtc();
      return DateTime.utc(
              utc.year, utc.month, utc.day, utc.hour, utc.minute, utc.second, utc.millisecond, utc.microsecond)
          .toIso8601String();
    }

    return dateTime.toIso8601String();
  }
}

date_time_serializer_test.dart

import 'package:test/test.dart';

void main() {
  test('correctly parses RFC 3339 dates', () {
    final cases = {
      // ISO 8601
      '2019-08-09T06:55:01.896826+05:00': DateTime.utc(2019, 8, 9, 1, 55, 1, 896, 826),
      '2019-08-09T06:55:01.896826Z': DateTime.utc(2019, 8, 9, 6, 55, 1, 896, 826),
      '2019-08-09T06:55:01Z': DateTime.utc(2019, 8, 9, 6, 55, 1),

      // RFC 3339
      '2019-05-31T10:35:45.347333481Z': DateTime.utc(2019, 5, 31, 10, 35, 45, 347, 333),
      '2018-01-29T18:39:51.396781283+01:00': DateTime.utc(2018, 1, 29, 17, 39, 51, 396, 781),
      '2019-08-09T06:55:01.8968264+00:00': DateTime.utc(2019, 8, 9, 6, 55, 1, 896, 826),
      '2020-10-20T20:55:01+09:00': DateTime.utc(2020, 10, 20, 11, 55, 1, 0, 0),
      '2020-10-20T20:35+09:00': DateTime.utc(2020, 10, 20, 11, 35, 0, 0, 0),
    };

    cases.forEach((source, datetime) {
      final deserialized = DateTimeSerializer.deserialize(source);
      print('source      :$source');
      print('datetime    :$datetime');
      print('deserialized:$deserialized');
      print('  tz    :${deserialized?.timeZoneName}');
      print('  offset:${deserialized?.timeZoneOffset}');
      print('  local :${deserialized?.toLocal()}');
      expect(deserialized, datetime);

      final serialized = DateTimeSerializer.serialize(deserialized);
      print('serialized  :$serialized');

      print('');
    });

    final now = DateTime.now();
    print('source      :$now');
    final serializedNow = DateTimeSerializer.serialize(now);
    print('serializedNow  :$serializedNow');
  });
}