Time Calculation in asp.net,C#

use array of button to make calculator then assign code on button click, on the base of index process the request.if you are not stick to server side programming you can use javaScript also to make nice calculator.
http://www.dapfor.com/en/net-suite/net-grid/tutorial/headers-and-columns

In general, you can find the time interval between two DateTime values by simply subtracting them. For example:


The TimeSpan structure has various properties to enable you to get the components of the interval or to express it as a certain number of units. For details see:

http://msdn.microsoft.com/en-us/library/system.timespan.aspx