May 04, 2012 09:33 AM|Young Yang - MSFT|LINK
If you want subtract 2 times, such as t3=t2-t1, you need to do this:
DateTime t1, t2;
t3 = t2.Subtract(t1);
var text = t3.ToString();
T3 needs to be a TimeSpan not a date time. The subtraction of two times is a time span not a time itself. as in 1:20 - 1:10 = 10 minutes. 10 minutes is not a time it is a time span.
Hope this helpful