TimeZoneInfo - same TimeZoneInfo id, different Daylight Savings support value

Question

We have a server which is failing to handle daylight saving time correctly wrt C# DateTime instances converting from UTC to local time. We are in the UK so have our servers set to "(UTC+00:00) Dublin, Edinburgh, Lisbon, London" region.

Symptom:

DateTime utcTime = new DateTime(2020, 10, 15, 23, 53, 20, DateTimeKind.Utc);
DateTime localTime = utcTime.ToLocalTime();
Console.WriteLine($"UTC time: {utcTime:s}");
Console.WriteLine($"Local time: {localTime:s}");

UTC time: 2020-10-15T23:53:20
Local time: 2020-10-15T23:53:20 // should be 2020-10-16T00:53:20

Investigation:

var gmt = TimeZoneInfo.FindSystemTimeZoneById("GMT Standard Time");
Console.WriteLine("GMT:");
Console.WriteLine($"ID: {gmt.Id}");
Console.WriteLine($"DisplayName: {gmt.DisplayName}");
Console.WriteLine($"DST: {gmt.SupportsDaylightSavingTime}");
var local = TimeZoneInfo.Local;
Console.WriteLine("Local:");
Console.WriteLine($"ID: {local.Id}");
Console.WriteLine($"DisplayName: {local.DisplayName}");
Console.WriteLine($"DST: {local.SupportsDaylightSavingTime}");

On "normal" workstations and servers, the output is:

GMT:
ID: GMT Standard Time
DisplayName: (UTC+00:00) Dublin, Edinburgh, Lisbon, London
DST: True

Local:
ID: GMT Standard Time
DisplayName: (UTC+00:00) Dublin, Edinburgh, Lisbon, London
DST: True

On the rogue server, it is:

GMT:
ID: GMT Standard Time
DisplayName: (UTC+00:00) Dublin, Edinburgh, Lisbon, London
DST: True

Local:
ID: GMT Standard Time
DisplayName: GMT Standard Time
DST: False

What could be the origin of this "GMT Standard Time" time zone info instance that has the same ID but a different name and DST support?

"rogue" server is Windows Server 2012 R2. I don't see anything obviously out of place in the registry keys

e.g. HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones\ has a "GMT Standard Time" key under it which declares the name "(UTC+00:00) Dublin, Edinburgh, Lisbon, London"

Under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation\ the TimeZoneKeyName value is "GMT Standard Time"

EDIT: We're running .NET framework code. The UTC-to-local conversion failure shows up in both .NET v2 and .NET v4.7.2.

user_8173870 · Answer

Your answer is right there in your outputs. You say the rouge server isn't supporting DST correctly and then when you inspect the time zone you see:

DST: False

Where the other servers show True.

You have two options, either:

Check the DST settings in code and add offset when it's not as expected
Enable DST on all servers to be the same

I would go with #2 because this is going to come up for all applications, not just yours. The server will be off by an hour for part of the year. So sometimes it's not a problem and then other times it is. Normally, you just hit the checkmark for enabling Daylight savings time in the clock settings. You cite registry values so here is the one for DST:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation

Value Name: DynamicDaylightTimeDisabled
Value Type: REG_DWORD
Value: 0

Setting DynamicDaylightTimeDisabled = 0 should make it so DST = True on this machine and it should then behave like the other ones.

TimeZoneInfo - same TimeZoneInfo id, different Daylight Savings support value

1 Answers