I don’t think this is possible using PostgreSQL alone in the most general case. When you install PostgreSQL, you pick a time zone. I’m pretty sure the default is to use the operating system’s timezone. That will usually be reflected in postgresql.conf as the value of the parameter “timezone”. But the value ends up as “localtime”. You can see this setting with the SQL statement.
But if you change the timezone in postgresql.conf to something like “Europe/Berlin”, then
show timezone; will return that value instead of “localtime”.
So I think your solution will involve setting “timezone” in postgresql.conf to an explicit value rather than the default “localtime”.