Interval to number

Postgres 7.2
I have an interval selected from a max(occurance) - min(occurance) where
bla.
I now want to multiply this by a rate - to create a charge...

If I use to_char( interval, 'SSSS');
I will get a seconds conversion - but that works on seconds since midnight -
hence
with a one day period.

Are there any better ways of converting a timestamp to an integer?


Thanks

Gareth