Your question is not clear enough. For subtracting datetime values please check the below examples.
DateTime dt = new DateTime();
dt.AddDays(-1);
dt.AddHours(-1);
dt.AddMinutes(-1);
dt.AddMonths(-1);
-or-
DateTime value1 = new DateTime(2011, 12, 20);
DateTime value2 = new DateTime(2011, 1, 1);
TimeSpan span = newYears.Subtract(value1);