Fix/datetime parser

lib/domain/media/media.dart

@@ -19,8 +19,8 @@ import 'package:webfeed/domain/media/status.dart';
 import 'package:webfeed/domain/media/text.dart';
 import 'package:webfeed/domain/media/thumbnail.dart';
 import 'package:webfeed/domain/media/title.dart';
-import 'package:webfeed/util/xml.dart';
 import 'package:webfeed/util/iterable.dart';
+import 'package:webfeed/util/xml.dart';
 import 'package:xml/xml.dart';
 
 class Media {

lib/domain/rss_content.dart

@@ -17,7 +17,7 @@ class RssContent {
   RssContent(this.value, this.images);
 
   factory RssContent.parse(XmlElement element) {
-    final dynamic? content = element.text;
+    final dynamic content = element.text;
     final images = <String>[];
     _imagesRegExp.allMatches(content).forEach((match) {
       images.add(match.group(1)!);

lib/util/datetime.dart

@@ -1,18 +1,26 @@
-import 'package:intl/intl.dart';
+import 'package:intl/intl.dart'; 
+import './timezone.dart';
+
+/// The `Z` part is not yet implemented according to https://pub.dev/documentation/intl/latest/intl/DateFormat-class.html
+/// We will remove it for now and parse the timezone separately.
+const rfc822DatePattern = 'EEE, dd MMM yyyy HH:mm:ss';
+final rfc822DateFormat = DateFormat(rfc822DatePattern, 'en_US');
 
-const rfc822DatePattern = 'EEE, dd MMM yyyy HH:mm:ss Z';
 
 DateTime? parseDateTime(dateString) {
   if (dateString == null) return null;
   return _parseRfc822DateTime(dateString) ?? _parseIso8601DateTime(dateString);
 }
 
+/// Try to parse `dateString` as an RFC 822 date.
+/// We will parse the date string as UTC and then 
+/// subtract the actual time offset from the parsed string.
 DateTime? _parseRfc822DateTime(String dateString) {
   try {
-    final num? length = dateString.length.clamp(0, rfc822DatePattern.length);
-    final trimmedPattern = rfc822DatePattern.substring(0, length as int?); //Some feeds use a shortened RFC 822 date, e.g. 'Tue, 04 Aug 2020'
-    final format = DateFormat(trimmedPattern, 'en_US');
-    return format.parse(dateString);
+    final localTime = rfc822DateFormat.parse(dateString, true);
+    final timezone = dateString.trim().split(' ').last;
+    final timeOffset = Duration(minutes: getTimeZoneOffset(timezone) ?? 0);
+    return localTime.subtract(timeOffset).toUtc();
   } on FormatException {
     return null;
   }

lib/util/timezone.dart

@@ -0,0 +1,37 @@
+const timeZoneAbbreviations = {
+  'EET': 2 *  60,
+  'CET': 1 * 60,
+  'GMT': 0,
+  'AST': -4 * 60,
+  'EST': -5 * 60,
+  'EDT': -4 * 60,
+  'CST': -6 * 60,
+  'CDT': -5 * 60,
+  'MST': -7 * 60,
+  'MDT': -6 * 60,
+  'PST': -8 * 60,
+  'PDT': -7 * 60,
+};
+
+/// Test this regexp online at https://regex101.com/r/mem3xt/1
+final offsetRegExp = RegExp(r'^(?<sign>[\+\-]?)(?<hours>\d{2})\:?(?<minutes>\d{2})$');
+
+
+/// Parse a potential timezone string and return the
+/// time offset in minutes
+int? getTimeZoneOffset(String timezone) {
+  // check if timezone is one of the known abbreviations
+  var offset = timeZoneAbbreviations[timezone.toUpperCase()];
+  if (offset != null) return offset;
+  // check if the timezone is of type offset
+  final match = offsetRegExp.firstMatch(timezone);
+  if (match != null) {
+    final sign = match.namedGroup('sign') == '-' ? -1 : 1;
+    // we know <hours> and <minutes> are not null because the RexExp matched
+    final hours = int.parse(match.namedGroup('hours')!);
+    final minutes = int.parse(match.namedGroup('minutes')!);
+    return sign * (60 * hours + minutes);
+  }
+
+  return null;
+}

test/datetime_test.dart

@@ -0,0 +1,89 @@
+import 'package:test/test.dart';
+import 'package:webfeed/util/datetime.dart';
+
+void main() {
+
+  group('RFC 822 date time', () {
+
+    test('parse GMT date time', () {
+      final dateString = 'Sat, 29 Apr 2023 12:00:00 GMT';
+      final result = parseDateTime(dateString);
+      expect(result, isNotNull);
+      expect(result!.isUtc, true);
+      expect(result, DateTime.utc(2023, 4, 29, 12, 0, 0));
+    });
+
+
+    test('parse EST date time', () {
+      final dateString = 'Sat, 29 Apr 2023 21:22:23 EST';
+      final result = parseDateTime(dateString);
+      expect(result, isNotNull);
+      expect(result!.isUtc, true);
+      expect(result, DateTime.utc(2023, 4, 30, 2, 22, 23));
+    });
+
+
+    test('parse +0000 offset date time', () {
+      final dateString = 'Fri, 28 Apr 2023 23:00:57 +0000';
+      final result = parseDateTime(dateString);
+      expect(result, isNotNull);
+      expect(result!.isUtc, true);
+      expect(result, DateTime.utc(2023, 4, 28, 23, 0, 57));
+    });
+
+
+    test('parse -0000 offset date time', () {
+      // yes, really, I saw this format here: https://feeds.megaphone.fm/bitcoinaudible
+      final dateString = 'Thu, 27 Apr 2023 19:17:00 -0000';
+      final result = parseDateTime(dateString);
+      expect(result, isNotNull);
+      expect(result!.isUtc, true);
+      expect(result, DateTime.utc(2023, 4, 27, 19, 17, 0));
+    });
+
+    test('parse +0100 offset date time', () {
+      final dateString = 'Fri, 28 Apr 2023 19:02:17 +0100';
+      final result = parseDateTime(dateString);
+      expect(result, isNotNull);
+      expect(result!.isUtc, true);
+      expect(result, DateTime.utc(2023, 4, 28, 18, 2, 17));
+    });
+
+
+    test('parse 02:00 offset date time', () {
+      final dateString = 'Thu, 27 Apr 2023 14:30:00 02:00';
+      final result = parseDateTime(dateString);
+      expect(result, isNotNull);
+      expect(result!.isUtc, true);
+      expect(result, DateTime.utc(2023, 4, 27, 12, 30, 0));
+    });
+
+
+    test('parse -0500 offset date time', () {
+      final dateString = 'Thu, 27 Apr 2023 14:30:00 -0500';
+      final result = parseDateTime(dateString);
+      expect(result, isNotNull);
+      expect(result!.isUtc, true);
+      expect(result, DateTime.utc(2023, 4, 27, 19, 30, 0));
+    });
+
+  });
+
+
+
+  group('ISO 8601 date time', () {
+
+    test('parse +00:00 offset date time', () {
+      final dateString = '2023-04-24T05:02:37+00:00';
+      final result = parseDateTime(dateString);
+      expect(result, isNotNull);
+      expect(result!.isUtc, true);
+      expect(result, DateTime.utc(2023, 4, 24, 5, 2, 37));
+    });
+
+  });
+
+
+
+
+}

test/rss_test.dart

@@ -73,7 +73,7 @@ void main() {
     expect(feed.items!.first.link, 'https://foo.bar.news/1');
     expect(feed.items!.first.guid, 'https://foo.bar.news/1?guid');
     expect(feed.items!.first.pubDate,
-        DateTime(2018, 03, 26, 14)); //Mon, 26 Mar 2018 14:00:00 PDT
+        DateTime.utc(2018, 03, 26, 21)); //Mon, 26 Mar 2018 14:00:00 PDT
     expect(feed.items!.first.categories!.first.domain, 'news');
     expect(feed.items!.first.categories!.first.value, 'Lorem');
     expect(feed.items!.first.author, 'alice@foo.bar.news');
@@ -104,7 +104,7 @@ void main() {
     expect(item.title, null);
     expect(item.link, 'http://www.foo.com');
     expect(item.pubDate,
-        DateTime(2001, 08, 27, 16, 08, 56)); //Mon, 27 Aug 2001 16:08:56 PST
+        DateTime.utc(2001, 08, 28, 0, 08, 56)); //Mon, 27 Aug 2001 16:08:56 PST
 
     expect(item.media!.group!.contents!.length, 5);
     expect(item.media!.group!.credits!.length, 2);

test/timezone_test.dart

@@ -0,0 +1,70 @@
+import 'package:test/test.dart';
+import 'package:webfeed/util/timezone.dart';
+
+main() {
+
+  group('Abbreviated timezones', () {
+
+    test('parse GMT timezone', () {
+      final offset = getTimeZoneOffset('GMT');
+      expect(offset, 0);
+    });
+
+    test('parse EET timezone', () {
+      final offset = getTimeZoneOffset('EET');
+      expect(offset, 2 * 60);
+    });
+
+    test('parse EST timezone', () {
+      final offset = getTimeZoneOffset('EST');
+      expect(offset, -5 * 60);
+    });
+
+  });
+
+
+  group('Offset timezones', () {
+
+    test('parse 00:00 timezone', () {
+      final offset = getTimeZoneOffset('00:00');
+      expect(offset, 0);
+    });
+
+    test('parse 0000 timezone', () {
+      final offset = getTimeZoneOffset('0000');
+      expect(offset, 0);
+    });
+
+    test('parse +01:00 timezone', () {
+      final offset = getTimeZoneOffset('+01:00');
+      expect(offset, 60);
+    });
+
+    test('parse 01:00 timezone', () {
+      final offset = getTimeZoneOffset('01:00');
+      expect(offset, 60);
+    });
+
+    test('parse 0100 timezone', () {
+      final offset = getTimeZoneOffset('0100');
+      expect(offset, 60);
+    });
+
+    test('parse -01:00 timezone', () {
+      final offset = getTimeZoneOffset('-01:00');
+      expect(offset, -60);
+    });
+
+    test('parse -0100 timezone', () {
+      final offset = getTimeZoneOffset('-0100');
+      expect(offset, -60);
+    });
+
+    test('parse -03:30 timezone', () {
+      final offset = getTimeZoneOffset('-03:30');
+      expect(offset, -3 * 60 - 30);
+    });
+
+  });
+
+}