DateTimeOffset adding TimeSpan returns invalid UTC offset for its TimeZoneInfo

Question

I am in the process of building a temporal expression library that must be properly globalized and therefore work in all available Time Zones.

Now I seem to be stuck as I'm not sure how to retrieve adjusted dates from DateTimeOffset objects in the correct Daylight Savings Time (DST) when the transition boundary is crossed using any variety of .Add to move days, hours, etc.

Interestingly, I figured out a work around for the local system's Time Zone but haven't found any way to apply the same strategy to any arbitrary Time Zone.

I was able to find a snippet (didn't keep source, sorry!) which tries to reverse lookup the Time Zone Info by offset but as there are multiple potential results, each of which likely have different DST rules that will not work. (There may be some optimizations available but the base premis is flawed I think)

public TimeZoneInfo GetTimeZoneInfo(DateTimeOffset Value)
{
    // Search available sytem time zones for a matching one
    foreach (var tzi in TimeZoneInfo.GetSystemTimeZones())
    {
        // Compare value offset with time zone offset
        if (tzi.GetUtcOffset(Value).Equals(Value.Offset))
        {
            return tzi;
        }
    }
}

This stuff can be a bit tedious to prove out so I've extracted the core issue into a couple methods and unit tests which will hopefully demonstrate the issue I'm facing.

public DateTimeOffset GetNextDay_Wrong(DateTimeOffset FromDateTimeOffset)
{
    // Cannot create a new DateTimeOffset using simply the supplied value's UtcOffset
    // because in PST, for example, it could be -7 or -8 depending on DST
    return new DateTimeOffset(FromDateTimeOffset.Date.AddDays(1), FromDateTimeOffset.Offset);
}

[TestMethod]
public void GetNextDay_WrongTest()
{
    var tz = TimeZoneInfo.FindSystemTimeZoneById("Pacific Standard Time");

    var workingDate = new DateTime(2009, 11, 2, 0, 0, 0);
    var failingDate = new DateTime(2009, 11, 1, 0, 0, 0);

    var workingDate_tz = new DateTimeOffset(workingDate, tz.GetUtcOffset(workingDate));
    var failingDate_tz = new DateTimeOffset(failingDate, tz.GetUtcOffset(failingDate));

    var actual_workingDate_tz = GetNextDay_Wrong(workingDate_tz);
    var actual_failingDate_tz = GetNextDay_Wrong(failingDate_tz);

    var expected_workingDate = new DateTime(2009, 11, 3, 0, 0, 0);
    var expected_failingDate = new DateTime(2009, 11, 2, 0, 0, 0);

    var expected_workingDate_tz = new DateTimeOffset(expected_workingDate, tz.GetUtcOffset(expected_workingDate));
    var expected_failingDate_tz = new DateTimeOffset(expected_failingDate, tz.GetUtcOffset(expected_failingDate));

    Assert.AreEqual(expected_workingDate_tz, actual_workingDate_tz, "Should have found the following day's midnight");
    Assert.AreEqual(expected_failingDate_tz, actual_failingDate_tz, "Failing date does not have the correct offset for it's DST");
}

public DateTimeOffset GetNextDay_LooksRight(DateTimeOffset FromDateTimeOffset)
{
    // Because we cannot create a new DateTimeOffset we simply adjust the one provided!
    var temp = FromDateTimeOffset;
    // Move back to midnight of the current day
    temp = temp.Subtract(new TimeSpan(temp.Hour, temp.Minute, temp.Second));
    // Now move to the next day
    temp = temp.AddDays(1);
    // Let the DateTimeOffset class do it's magic
    temp = temp.ToLocalTime();
    // Check if the time zone has changed
    if (FromDateTimeOffset.Offset != temp.Offset)
    {
        // Calculate the change amount (could be 30 mins or even stranger)
        var delta = FromDateTimeOffset.Offset - temp.Offset;
        // Adjust the temp value by the delta
        temp = temp.Add(delta);
    }
    return temp.ToLocalTime();
}

[TestMethod]
public void GetNextDay_LooksRightTest()
{
    // Everything is looking good and the test passes now, so we're home free yeah?

    // { To work this needs to match your system's configured Local Time Zone, I'm in PST }
    var tz = TimeZoneInfo.FindSystemTimeZoneById("Pacific Standard Time");

    var workingDate = new DateTime(2009, 11, 2, 0, 0, 0);
    var failingDate = new DateTime(2009, 11, 1, 0, 0, 0);

    var workingDate_tz = new DateTimeOffset(workingDate, tz.GetUtcOffset(workingDate));
    var failingDate_tz = new DateTimeOffset(failingDate, tz.GetUtcOffset(failingDate));

    var actual_workingDate_tz = GetNextDay_LooksRight(workingDate_tz);
    var actual_failingDate_tz = GetNextDay_LooksRight(failingDate_tz);

    var expected_workingDate = new DateTime(2009, 11, 3, 0, 0, 0);
    var expected_failingDate = new DateTime(2009, 11, 2, 0, 0, 0);

    var expected_workingDate_tz = new DateTimeOffset(expected_workingDate, tz.GetUtcOffset(expected_workingDate));
    var expected_failingDate_tz = new DateTimeOffset(expected_failingDate, tz.GetUtcOffset(expected_failingDate));

    Assert.AreEqual(expected_workingDate_tz, actual_workingDate_tz, "Should have found the following day's midnight");
    Assert.AreEqual(expected_failingDate_tz, actual_failingDate_tz, "Failing date does not have the correct offset for it's DST");
}

[TestMethod]
public void GetNextDay_LooksRight_FAILTest()
{
    // Here is where the frustrating part is... aparantly the "magic" that DateTimeOffset provides only works for your systems Local Time Zone...

    // { To properly fail this cannot match your system's configured Local Time Zone, I'm in PST so I use EST }
    var tz = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");

    var workingDate = new DateTime(2009, 11, 2, 0, 0, 0);
    var failingDate = new DateTime(2009, 11, 1, 0, 0, 0);

    var workingDate_tz = new DateTimeOffset(workingDate, tz.GetUtcOffset(workingDate));
    var failingDate_tz = new DateTimeOffset(failingDate, tz.GetUtcOffset(failingDate));

    var actual_workingDate_tz = GetNextDay_LooksRight(workingDate_tz);
    var actual_failingDate_tz = GetNextDay_LooksRight(failingDate_tz);

    var expected_workingDate = new DateTime(2009, 11, 3, 0, 0, 0);
    var expected_failingDate = new DateTime(2009, 11, 2, 0, 0, 0);

    var expected_workingDate_tz = new DateTimeOffset(expected_workingDate, tz.GetUtcOffset(expected_workingDate));
    var expected_failingDate_tz = new DateTimeOffset(expected_failingDate, tz.GetUtcOffset(expected_failingDate));

    Assert.AreEqual(expected_workingDate_tz, actual_workingDate_tz, "Should have found the following day's midnight");
    Assert.AreEqual(expected_failingDate_tz, actual_failingDate_tz, "Failing date does not have the correct offset for it's DST");
}

Answer 1

The underlying issue with this case is that there is not enough information to perform the appropriate Time Zone conversions. Simple offset is not specific enough to infer the Time Zone because for a given offset at a specific moment there can be multiple potential Time Zones. Because the DateTimeOffset object does not capture and maintain the information about the Time Zone it was created with it must be the responsibility of something external to that class to maintain this relationship.

The thing that threw me off the scent was the call ToLocalTime() which I later realized was in fact introducing an implied TimeZoneInfo into the calculation, that of the configured local time for the machine and I believe that internally the DateTimeOffset must be converting to UTC by simply removing the configured offset and then creating a new DateTimeOffset class using the constructor taking DateTime [in UTC] and TimeZoneInfo [from local system] to produce the correct dst aware resulting date.

Given this limitation I no longer see any value in the DateTimeOffset class over the equally accurate and more valuable combination of DateTime and TimeZoneInfo.