Does DDay Ical library calculate recurring rules correctly?

Question

I've run some tests. Here is my code:

var systemTimeZone = TimeZoneInfo.FindSystemTimeZoneById("Pacific Standard Time");
var icalTimeZone = iCalTimeZone.FromSystemTimeZone(systemTimeZone);

var startTimeSearch = new DateTime(2015, 9, 8, 0, 0, 0, DateTimeKind.Utc);
var endTimeSearch = new DateTime(2015, 12, 1, 00, 0, 0, DateTimeKind.Utc);

var iCalendar = new iCalendar();
var pacificTimeZone = _iCalendar.AddTimeZone(icalTimeZone);

var event = new Event
{
    Summary = "This is an event at 2015-09-08 10:30 PST (2015-09-08 17:30 UTC)",
    DTStart = new iCalDateTime(2015, 9, 8, 10, 30, 0, pacificTimeZone.TZID, iCalendar),
    Duration = new TimeSpan(0, 1, 0, 0)
};

var rp = new RecurrencePattern("FREQ=WEEKLY;UNTIL=20151112T080000Z;WKST=SU;BYDAY=TU");

event.RecurrenceRules.Add(rp);
iCalendar.Events.Add(_event);
var occurrences = iCalendar.GetOccurrences(startTimeSearch, endTimeSearch);

---

Here's the result:

10 occurences - here are the occurences' Period.StartTime.Value and UTC properties

Value - 9/8/2015 10:30:00 - UTC 9/8/2015 17:30:00
Value - 9/15/2015 10:30:00 - UTC 9/15/2015 17:30:00
Value - 9/22/2015 10:30:00 - UTC 9/22/2015 17:30:00
Value - 9/29/2015 10:30:00 - UTC 9/29/2015 17:30:00
Value - 10/6/2015 10:30:00 - UTC 10/6/2015 17:30:00
Value - 10/13/2015 10:30:00 - UTC 10/13/2015 17:30:00
Value - 10/20/2015 10:30:00 - UTC 10/20/2015 17:30:00
Value - 10/27/2015 10:30:00 - UTC 10/27/2015 17:30:00
Value -11/3/2015 10:30:00 - UTC 11/3/2015 17:30:00 (THIS SHOULD BE 18:30:00!)
Value - 11/10/2015 10:30:00 - UTC 11/10/2015 17:30:00 (THIS SHOULD BE 18:30:00!)

As you can see, the last two should have a UTC time of 18:30:00. So I'm wondering if this is just the way DDay Ical works and I shouldn't count on it getting the UTC datetime right, or if I'm doing something wrong.

Answer 1

You are getting this time difference due to daylight savings. One possible solution is not to get the time zone by name Pacific Standard Time

 // First load a file containing time zone information for Pacific Standard Time
var systemTimeZone = TimeZoneInfo.FindSystemTimeZoneById("Pacific Standard Time");

In place of this use system time zone like

var systemTimeZone = TimeZoneInfo.GetSystemTimeZones().FirstOrDefault();

Or simply add the local timezone:

iCalendar iCalendar= new iCalendar();
iCalendar.AddLocalTimeZone();

To find all registered timezones, click here

ReadOnlyCollection<TimeZoneInfo> zones = TimeZoneInfo.GetSystemTimeZones();
Console.WriteLine("The local system has the following {0} time zones", zones.Count);
foreach (TimeZoneInfo zone in zones)
   Console.WriteLine(zone.Id);

I simply use GetSystemTimeZones() in my code like

public static void Test1()
    {
        var systemTimeZone = TimeZoneInfo.GetSystemTimeZones().FirstOrDefault();
        //TimeZoneInfo.FindSystemTimeZoneById("Pacific Standard Time");
        var icalTimeZone = iCalTimeZone.FromSystemTimeZone(systemTimeZone);

        var startTimeSearch = new DateTime(2015, 9, 8, 0, 0, 0, DateTimeKind.Utc);
        var endTimeSearch = new DateTime(2015, 12, 1, 00, 0, 0, DateTimeKind.Utc);

        var iCalendar = new iCalendar();
        var pacificTimeZone = iCalendar.AddTimeZone(icalTimeZone);

        var _event =
        new Event
        {
            Summary = "This is an event at 2015-09-08 10:30 PST (2015-09-08 17:30 UTC)",
            DTStart = new iCalDateTime(2015, 9, 8, 10, 30, 0, pacificTimeZone.TZID, iCalendar),
            Duration = new TimeSpan(0, 1, 0, 0)
        };

        var rp = new RecurrencePattern("FREQ=WEEKLY;UNTIL=20151112T080000Z;WKST=SU;BYDAY=TU");

        _event.RecurrenceRules.Add(rp);
        iCalendar.Events.Add(_event);

        var occurrences = iCalendar.GetOccurrences(startTimeSearch, endTimeSearch);
    }

when I debug it got got output like

you can change the time zone accordingly and get the result. Hope it helps you.

Answer 2

Don't use dday.ical; use ical.net.

This should get you started:

var iCalendar = new Calendar();

var start = new DateTime(2015, 9, 8, 10, 30, 0);
var e = new Event
{
    Summary = "This is an event at 2015-09-08 10:30 PST (2015-09-08 17:30 UTC)",
    DtStart = new CalDateTime(start, "Pacific Standard Time"),
    Duration = TimeSpan.FromHours(1)
};

var rp = new RecurrencePattern("FREQ=WEEKLY;UNTIL=20151112T080000Z;WKST=SU;BYDAY=TU");
e.RecurrenceRules.Add(rp);
iCalendar.Events.Add(e);

var startTimeSearch = new DateTime(2015, 9, 8, 0, 0, 0, DateTimeKind.Utc);
var endTimeSearch = new DateTime(2015, 12, 1, 00, 0, 0, DateTimeKind.Utc);
var occurrences = iCalendar.GetOccurrences(startTimeSearch, endTimeSearch);

Results: