You need to perform calculations with dates and times.
Use the TimeSpan and DateTime structures, both of which provide Add and Subtract methods.
The .NET Framework provides two structures for manipulating date and time information. The DateTime structure stores a reference to a single date and time (such as January 20, 2004 at 12:00 AM). The TimeSpan structure stores an interval of time (such as three hours). TimeSpan is ultimately measured in ticks (a unit of time equal to 100 nanoseconds), and DateTime is stored as the number of ticks since 12:00:00 midnight, January 1, 0001 C.E. The greater a DateTime is, the later the date. The greater a TimeSpan is, the larger the interval of time.