It is that easy if they are on the same row.... which I think is what you want to do. That will give you days between dates. Often conversions or functions and such can help and are used with dates. I use DATEDIFF function in SQL Server while Oracle and stuff has there own. Don't know of an ANSI standard.
Edit:whoops... forgot the time was a part of your Q.... Dunno bro. hmm... ? Sorry.