Here's a simple query you can either wrap in a function or cut and paste all over the place:
select now()::timestamp(0), (extract(epoch from now()::timestamptz(0)-date_trunc('d',now()))::int)/60;
It'll give you the current time, and a number from 0 to the n-1 where n=60 here. To make it every 5 minutes, make that number 300 and so on. It groups by the seconds since the start of the day. To make it group by seconds since year begin, hour begin, or whatever else, change the 'd' in the date_trunc.
