Impact of OS timezone change on Postgres Database
If you have set a time zone for your Postgres DB cluster in postgresql.conf, changing the time zone of the server OS will have no bearing on your databases at all - as long as you don't actually change the time itself, not just the time zone.

You need to make sure that none of your client programs feed local time to the database, which would actually change with the time zone setting of the server.

Even the time zone of the Postgres server has no bearing on data while operating with the data type timestamptz (timestamp with time zone). Only the display is adjusted to the timezone setting of each session - which defaults to the setting of the DB cluster unless specified otherwise. But the value is always the same.

However, it matters for timestamp (timestamp without time zone). And it also matters whenever a timestamptz is cast to timestamp or date (and the time zone is not provided explicitly with the AT TIME ZONE construct). This may even be hidden in expressions transforming or formatting temporal data. All of those will show an actual time-shift of two hours when the timezone setting of the session changes.

Related:


PostgreSQL timezone does not match system timezone
Ignoring timezones altogether in Rails and PostgreSQL