There is an article in Microsoft site http://support.microsoft.com/support/kb/ that explains this.
Look for Article ID: Q115055


>Its not only storing time in field and doing the calculation.
>In my new to be project for vehical maintenance workshop I may need to give many user input time fields which will be stored to database and used for various calculation. Storing in datetime format is easy but geting time from user is difficult in that format is there any class which help get time in normal HH:MM and convert it to seconds.
>Rajesh