IsDaylightSavingTime is triggered at 1:00 instead of 2:00

Question

I'm simply trying to determine if a given date falls within Daylight Saving Time. In 2017, Nov 5th at 2:00 AM should be the trigger.

If my input is 11/5/17 00:00:00 the output is true for DST
If my input is 11/5/17 01:00:00 the output is false for DST

I would have expected 1 AM to be true, and 2 AM to be false for DST

here is my code

var dateTime = new DateTime(2017,11,5,0,0,0);
var targetTimeZone = TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time");

if (targetTimeZone.IsDaylightSavingTime(dateTime)) {
    Console.WriteLine("Daylight Saving Time");
}

Consider that 1am happens twice. You need to tell it which one it is when you create your `DateTime`. — James Thorpe, Jan 03 '18 at 14:12
`IsAmbiguousTime` will help you *reason* about this more clearly but if all you have is a `DateTime` that isn't `Utc` there's not much that can be done about it - nobody has any way of knowing which point in time that `Local` datetime value represented. — Damien_The_Unbeliever, Jan 03 '18 at 14:17
Use TimeZoneInfo.IsAmbiguousTime() to ensure you should not be asking this question. — Hans Passant, Jan 03 '18 at 14:17
What about if you store somewhere XML, DB, etc. the previous DST in order to know and check later. — Federico Navarrete, Jan 03 '18 at 14:22
Stuff like this is *precisely* why NodaTime was created. You might consider looking into it. — , Jan 03 '18 at 14:35
Consider using UTC time in your database and change it on display to the user to the local time. — Ctznkane525, Jan 03 '18 at 15:13

score 1 · Accepted Answer · answered Jan 14 '18 at 18:41

From the documentation (in the Remarks section):

If the dateTime parameter specifies an ambiguous time in the current object's time zone, the TimeZoneInfo.IsDaylightSavingTime method interprets dateTime as standard time and returns false if its Kind property is DateTimeKind.Local or DateTimeKind.Unspecified. If the Kind property is DateTimeKind.Utc, this method will select the correct ambiguous time and indicate whether it is a daylight saving time.

It then goes on to explain IsAmbiguousTime, and a long example showing how to use them that very closely matches the scenario you described.

Also note that the overload for IsDaylightSavingTime that accepts a DateTimeOffset parameter does not have this issue because values cannot be ambiguous when an offset is present - similar to when Kind is UTC.

IsDaylightSavingTime is triggered at 1:00 instead of 2:00

1 Answers1