One word of advice -- when I first started to use DateTime I was confused by the fact that the difference of two DateTime objects was not directly convertible into a number of seconds.

What you'll probably want to do is use the ->utc_rd_as_seconds method to first convert the DateTime objects to seconds and then perform the subtraction. Alternatively, subtract_datetime_absolute might work, too, although the former method also ignores leap seconds which may be helpful.