Can anyone suggest a good way to store and then calculate elapsed time in hrs or minutes.

I currently have two date fields date_started and date_completed, both are date types.
1. Is this a good way to store start/stop times?
3. How do you calculate elapsed time in either minutes or hrs?

My first attempt was to take the difference of the two, such as
select date_completed - date_started.
But this appears to provide the difference in days.
I've also tried some to_num(to_char combinations but have not had any success.

Any suggestions would be greatly appreciated.

Thanks,
-Isaac