Postgres: how do you round a timestamp up or down to the nearest minute?

Use the built-in function date_trunc(text, timestamp), for example:

select date_trunc('minute', now())

Edit: This truncates to the most recent minute. To get a rounded result, add 30 seconds to the timestamp first, for example:

select date_trunc('minute', now() + interval '30 second')

This returns the nearest minute.

See Postgres Date/Time Functions and Operators for more info