I originally asked this question so many years ago, I don't even remember doing it. But since then, all my database date/time code on the client side has been greatly simplified. The trick is to tell PostgreSQL the local time zone when the DB connection is first established, and let the server automatically add or remove the necessary hours/minutes when it sends back timestamps. This way, timestamps are always in local time.
You do that with a 1-time call similar to this one:
SET SESSION TIME ZONE 'Europe/Berlin';
You can also use one of the many timezone abbreviations. For example, these two lines are equivalent:
SET SESSION TIME ZONE 'Asia/Hong_Kong';
SET SESSION TIME ZONE 'HKT';
The full list of timezones can be obtained with this:
SELECT * FROM pg_timezone_names ORDER BY name;
Note: there are over 1000 timezone names to pick from!
I have more details on PostgreSQL and timezones available on this post: https://www.ccoderun.ca/programming/2017-09-14_PostgreSQL_timestamps/index.html