Datetime Converting two times not equal starting time

Question

I'm using the TimeZoneConverter NuGet package.

The Pacific standart time is use UTC-8

1. Created a datetime as pacific timezone.
2. The pacific time converted utc time.
3. The utc time converted again to pacific timezone. But the result doesnt equal datetime in the first item. Why? Any idea?

[Fact]
public void DateTimeConvertTesting()
{
    TimeZoneInfo pacificTimeZone = TZConvert.GetTimeZoneInfo("Pacific Standard Time"); // TimeZoneConverter
    DateTimeOffset dateTimeOffsetPacific2 = new DateTimeOffset(2021, 09, 01, 11, 59, 0, pacificTimeZone.BaseUtcOffset);
    DateTime utcDateTime = dateTimeOffsetPacific2.UtcDateTime;
    DateTime pacificTime = TimeZoneInfo.ConvertTimeFromUtc(utcDateTime, pacificTimeZone);
    Assert.True(dateTimeOffsetPacific2.DateTime == pacificTime);
}

Answer 1

I think Caius Jard hits the correct issue on the comment.

Looks like for Pacific Standard Time on 2021, DST started March 14 02:00 AM and ends 7 November 02:00 AM.

TimeZoneInfo.ConvertTimeFromUtc method handles the daylight saving itself. That means, if the Datetime or DateTimeOffset is a DST on Pacific Standard Time, it returns the DST added value (which is 1 hour usually). That's why your pacificTime variable will be {9/1/2021 12:59:00 PM} not {9/1/2021 11:59:00 PM}

If you have to use ConvertTimeFromUtc for your test, it would be better to check first your DateTime or DateTimeOffset is a daylight saving time or not for that specific timezone. You can use TimeZoneInfo.IsDaylightSavingTime method for that which have overloads both DateTime and DateTimeOffset.

var isDaylightSavingTime = pacificTimeZone.IsDaylightSavingTime(utcDateTime);
// for 2021, 09, 01, 11, 59, 0, this returns true.