Dart DateTime DateFormat to RFC-822 String

formatRfc822.dart

import 'package:intl/intl.dart';
import 'package:test/test.dart';

formatRfc822(DateTime d) {
    var template = "EEE, dd MMM yyyy HH:mm:ss";
    var out = DateFormat(template).format(d);
    if (d.isUtc) {
      out += ' GMT';
    } else {
      var offset = d.toLocal().timeZoneOffset.inHours * 100;
      out += ' +${offset < 1000 ? '0' : ''}$offset';
    }
    return out;
  } 

main(){
  test('rfc 822', () {
    var d = DateTime(2020, 4, 21, 20);
    expect(formatRfc822(d.toUtc()), 'Tue, 21 Apr 2020 18:00:00 GMT');
    expect(formatRfc822(d.toLocal()), 'Tue, 21 Apr 2020 20:00:00 +0200');
  });
}