I want to subtract a time-span from a date-time object. Date is 1983/5/1 13:0:0 (y/m/d-h:m:s) Time span is 2/4/28-2:51:0 (y/m/d-h:m:s) I can use the native DateTime and TimeSpan objects to do this, after converting years and months of the time-span to days (assuming a 30 day month and a ~364 day year). new DateTime(1981,5,1,13,0,0).Subtract(new TimeSpan(878,13,51,0)); With this i get the result: {12/4/1978 11:09:00 PM} But this is not exactly what i expected. If i do this manually (assuming a 30 day month) i get 1981/0/3-10:9:0 This is pretty close to what i'm after except i shouldn't get 0 for month and year should be 1980. So can someone please show me how i can do this manually and avoid getting a 0 month value? Also why do i get a completely different value when i use native classes?