By default postgres should be in UTC , show timezone; if it isn't change configuration timezone = 'UTC' in postgresq.conf
Store dates always in UTC, no mather user timezone, use AT TIME ZONE '' to convert it your query.
To know a list of valid values select * from pg_timezone_names;
to experiment : set timezone='Europe/Berlin' ; SELECT NOW(); SELECT NOW() AT TIME ZONE 'Europe/Berlin' SELECT NOW() AT TIME ZONE 'UTC'
read http://justatheory.com/computers/databases/postgresql/use-timestamptz.html