.NET TimeZoneInfo wrong about Daylight savings

5
votes

Can anyone help make sense of this. Microsoft’s TimeZoneInfo class in 3.5 is telling me the following GMT date is not in Daylight savings for the Eastern timezone, but it is. 

// Get Eastern Timezone
TimeZoneInfo tzEasternZone = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");

// Convert to EST 
DateTime easternTime = TimeZoneInfo.ConvertTime(DateTime.Parse("2009-11-01T05:00:00Z"), tzEasternZone);

// Daylight saving IS in effect on this date, but Microsoft doesn't think so
Boolean isDaylight = easternTime.IsDaylightSavingTime();

Here are 2 websites saying it is in daylight savings:
http://www.timeanddate.com/worldclock/converted.html?month=11&day=1&year=2009&hour=5&min=0&sec=0&p1=0&p2=198
http://www.timezoneconverter.com/cgi-bin/tzc.tzc

3
.nettimezonedst
what operating system are you testing this on? – x0n

3 Answers

6
votes

See: TimeZoneInfo.IsDaylightSaving

Because the TimeZoneInfo.IsDaylightSavingTime(DateTime) method can return false for a date and time that is ambiguous (that is, a date and time that can represent either a standard time or a daylight saving time in a particular time zone), the TimeZoneInfo.IsAmbiguousTime(DateTime) method can be paired with the IsDaylightSavingTime(DateTime) method to determine whether a time may be a daylight saving time. Because an ambiguous time is one that can be both a daylight saving time and a standard time...

Also you might look at this...

TimeZoneInfo.GetAmbiguousTimeOffsets Method

Returns information about the possible dates and times that an ambiguous date and time can be mapped to.

5
votes

This will work:

TimeZoneInfo tzEasternZone = TimeZoneInfo.FindSystemTimeZoneById(
                                          "Eastern Standard Time");

DateTime utc = DateTime.Parse("2009-11-01T05:00:00Z",
                              CultureInfo.InvariantCulture,
                              DateTimeStyles.RoundtripKind);

bool isDaylight = tzEasternZone.IsDaylightSavingTime(utc);

The original code had two issues:

  • Even though a UTC value was being provided, it was getting converted to local kind in the Parse statement. So ambiguity could be introduced there.

  • The IsDaylightTime method on the DateTime class will assume the local time zone if the kind is local or unspecified. After calling ConvertTime, the result has unspecified kind, so it was checking against the rules of the local time zone, not the eastern time zone.

2
votes

TimeZoneInfo.GetUtcOffset(DateTime)

Correctly returns the offset with daylight savings factored in if the specified date is inside the period