TimeZoneInfo conversion of DateTimeOffset fails-CodePudding

In trying to convert a DateTimeOffset between two time zones, I've successfully used TimeZoneInfo.ConvertTime(), but I found a case where the conversion doesn't work. That is - conversion between UTC 11.00 and UTC 10:30 which has no effect.

Here is my completely vanilla source:

public static DateTimeOffset ConvertToNewTimeZone(this DateTimeOffset dt, string newTz)
{
  var tzi = DeduceTimeZone(newTz); // converts out of proprietary format into std format
  var result = TimeZoneInfo.ConvertTime(dt, tzi);
  return result;
}

And here's a debug view of the values of my variables.

Notice how the result is still in UTC 11:00. I expected the result to be in 10:30. Can anyone explain why the conversion didn't happen? I'm not aware of any adjustments that might apply.

CodePudding user response：

Because of the daylight saving time (DST), you may know that January is summer in the southern hemisphere where Lord Howe Island is located, if you try to convert a different date in winter, the offset will be 10:30.

var dt = new DateTime(2022, 7, 1);
TimeZoneInfo.ConvertTime(new DateTimeOffset(dt), tzi);
// 22/7/1 02:30:00  10:30

TimeZoneInfo.GetUtcOffset

Time zone essentials

CodePudding user response：

In the end I narrowed it down when I noticed that the implementation of the TimeZoneInfo in Windows is different from that in Linux and MacOS. My tests were inadvertently comparing local time (debugging on windows) with a different set of time zones (where I was logging from a Docker container running on AKS on Azure, which was a Linux container).

In the end, it appears that my time zone conversion defaulted to UTC, because the System Timezone Database is empty. That is, TimeZoneInfo.GetSystemTimeZones().Count == 0.

Not sure what to do about it, but at least I know now why my code was failing.