Creating subsets on timestamp with modulo, date_trunc and ?suggestions?

A simple way I came up is to truncate the date. So if you have 2009-08-08,
and you want a subset on month, then just truncate the day-part: 2009-08-00
on the whole column, and SELECT DISTINCT so you have a subset. You can use
this subset then to join the dates, GROUP BY and aggregate....

An other way I found to do this is in Celko's 'SQL for smarties'. He uses
modulo there. It seems powerful, but also tricky to implement.

I was wondering if anyone knew some other way to create a subset of a
timestamp column. Any input is welcome.

Regards,
Davor