Selecting between dates

Hi,

I am creating a script that sends out reminders to subscribers to a site who
have not paid.  I can't figure out the math that will add 7 days to a date.

I want something like this:

select * from company where created
is between (lastpaid + 14 days) and (created + 28 days);

I can't figure out the syntax that will let me add days to a date.  I checked
the archives and the Postgresql doc's, but nothing.

Ideas?

Thanks,

Wim Kerkhoff.