added timezone support to events

lib/src/event.dart

@@ -7,7 +7,9 @@ import 'utils.dart' as utils;
 class IEvent extends ICalendarElement with EventToDo {
   IEventStatus status;
   DateTime start;
+  String? startTimeZone;
   DateTime? end;
+  String? endTimeZone;
   Duration? duration;
   ITimeTransparency? transparency = ITimeTransparency.OPAQUE;
 
@@ -23,7 +25,9 @@ class IEvent extends ICalendarElement with EventToDo {
     String? uid,
     this.status = IEventStatus.CONFIRMED,
     required this.start,
+    this.startTimeZone,
     this.end,
+    this.endTimeZone,
     this.duration,
     String? summary,
     String? description,
@@ -61,11 +65,13 @@ class IEvent extends ICalendarElement with EventToDo {
     if ((end == null && duration == null)) {
       out.writecrlf('DTSTART;VALUE=DATE:${utils.formatDate(start)}');
     } else {
-      out.writecrlf('DTSTART:${utils.formatDateTime(start)}');
+      out.writecrlf(
+          'DTSTART${utils.injectTimeZone(start, startTimeZone)}:${utils.formatDateTime(start)}');
     }
 
     if (end != null) {
-      out.writecrlf('DTEND:${utils.formatDateTime(end!)}');
+      out.writecrlf(
+          'DTEND${utils.injectTimeZone(end!, endTimeZone)}:${utils.formatDateTime(end!)}');
     }
     if (duration != null) {
       out.writecrlf('DURATION:${utils.formatDuration(duration!)}');

lib/src/utils.dart

@@ -1,8 +1,25 @@
 import 'package:intl/intl.dart';
 
+DateFormat utcDateTimeFormatter = DateFormat("yyyyMMdd'T'HHmmss'Z'");
+DateFormat floatingDateTimeFormatter = DateFormat("yyyyMMdd'T'HHmmss");
+
 formatDateTime(DateTime dt) => dt.isUtc
-    ? DateFormat("yyyyMMdd'T'HHmmss'Z'").format(dt)
-    : DateFormat("yyyyMMdd'T'HHmmss").format(dt);
+    ? utcDateTimeFormatter.format(dt)
+    : floatingDateTimeFormatter.format(dt);
+
+/// If this DateTime is a UTC date time or the time zone is null this returns an empty string.
+/// If a time zone string is passed, returns a TZID parameter.
+/// It is assumed that the time zone string is from a unique database therefore it is prepended with a /
+///
+/// see https://icalendar.org/iCalendar-RFC-5545/3-2-19-time-zone-identifier.html
+String injectTimeZone(DateTime dt, String? timeZone) {
+  if (dt.isUtc || timeZone == null) {
+    return "";
+  } else {
+    return ";TZID=/$timeZone";
+  }
+}
+
 formatDate(DateTime dt) => DateFormat("yyyyMMdd").format(dt);
 formatDuration(Duration d) {
   String out = (d.isNegative ? '-' : '+') + 'P';

test/event_test.dart

@@ -53,6 +53,17 @@ main() {
       print(e.serialize());
       print(e2.serialize());
     });
+
+    test("with time zone", () {
+      IEvent event = IEvent(
+        start: DateTime(2022, 2, 2, 10),
+        startTimeZone: "Europe/Berlin",
+        end: DateTime(2022, 2, 2, 19),
+        endTimeZone: "Europe/Berlin",
+      );
+      String result = event.serialize();
+      print(result);
+    });
     // TODO create tests
   });
 }

test/utils_test.dart

@@ -13,6 +13,21 @@ main() {
       expect(utils.formatDateTime(DateTime(1948, 12, 30, 12, 45, 33)),
           '19481230T124533');
     });
+    test('injectTimeZone', () {
+      expect(utils.injectTimeZone(DateTime.utc(2000, 2, 1, 23, 1), null), '');
+      expect(utils.injectTimeZone(DateTime(2000, 2, 1, 23, 1), null), '');
+      expect(
+          utils.injectTimeZone(
+              DateTime(1948, 12, 30, 12, 45, 33), "Europe/Berlin"),
+          ';TZID=/Europe/Berlin');
+    });
+    test('formatDateTime with UTC time', () {
+      expect(utils.formatDateTime(DateTime.utc(2000, 2, 1, 23, 1)),
+          '20000201T230100Z');
+      expect(utils.formatDateTime(DateTime.utc(1948, 12, 30, 12, 45, 33)),
+          '19481230T124533Z');
+    });
+
     test('formatDuration', () {
       expect(
           utils.formatDuration(