Question

I encountered what may be a leap year in .NET’s DateTime handling, specifically ToLocalTime(). Here’s some code which reproduces the problem (I’m in the Pacific time zone):

DateTime dtStartLocal = DateTime.Parse('2009-02-28T23:00:00.0-08:00'); DateTime dtEndLocal = dtStartLocal.AddYears(3); DateTime dtStartUtc = dtStartLocal.ToUniversalTime(); DateTime dtEndUtc = dtStartUtc.AddYears(3); DateTime dtEndLocal2 = dtEndUtc.ToLocalTime(); DateTime dtStartLocal2 = dtStartUtc.ToLocalTime(); Console.WriteLine('START: 1={0}, 2={0}', dtStartLocal, dtStartLocal2); Console.WriteLine('END  : 1={0}, 2={1}', dtEndLocal, dtEndLocal2); Console.ReadLine(); 

The output is:

START: 1=2/28/2009 11:00:00 PM, 2=2/28/2009 11:00:00 PM
END : 1=2/28/2012 11:00:00 PM, 2=2/29/2012 11:00:00 PM

Notice the variable which I did ToUniversalTime().AddYears(3).ToLocalTime() is different than just AddYears(3), it’s one day ahead.

Has anyone encountered this? If this is expected, can someone explain the logic behind it?

NOTE: Yes, the best approach is to work entirely in UTC and not flip flop between them. This isn’t something which is effecting me, but a peculiarity I encountered. Essentially I misunderstood how AddYears() worked and now I can see why it’s doing what it’s doing (see my selected answer below).

Answer 1

I think that this is working correctly.

DateTime dtStartUtc = dtStartLocal.ToUniversalTime(); 

PST is UTC-8. Therefore, this converts the time to March 1, 2009, 07:00:00.

DateTime dtEndUtc = dtStartUtc.AddYears(3); 

This adds three years to the previous time, putting it at March 1, 2012, 07:00:00.

DateTime dtEndLocal2 = dtEndUtc.ToLocalTime(); 

This converts the end time back to PST, which would be February 29, 2012, 11:00:00.

I’d say this is just a side affect of converting between local and UTC time.