Keep in mind that SECONDS() function returns elapsed time since midnight. you've to make ajustment for that if process crosses midnight. Take also a look at Show seconds in a readable format FAQ #13091.

>In addition to the other answers about the datetimes calculation, you could run the SECONDS() function at the start of the process and save it to a var, then run it again at the end of the process. The advantage with this method is that SECONDS() works in number of milliseconds, so if you have to measure processes that are very short, this would be the way to go.