Fix parsing datetime

lib/util/datetime.dart

@@ -1,21 +1,26 @@
 import 'package:intl/intl.dart';
+import 'timezone.dart';
 
-const rfc822DatePattern = 'EEE, dd MMM yyyy HH:mm:ss Z';
+/// 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');
 
 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;
+}