The part that keeps biting teams is that AT TIME ZONE is a cast, not an annotation. On a timestamp it means "interpret these wall-clock digits as this zone and produce a timestamptz". On a timestamptz it means "render this instant as wall-clock digits in this zone and produce a timestamp". Same syntax, opposite direction, and the session TimeZone is the hidden third argument.

That is also why calendar math and instant math disagree. interval '1 day' on a timestamptz is 24 hours, so a 09:00 local appointment drifts across DST. The usual fix is to strip to timestamp in the civil zone, add the calendar interval, then cast back. The double AT TIME ZONE 'UTC' in the post is that pattern with UTC as the civil zone, which only works if the civil zone really is UTC.

What I have settled on: store events as timestamptz, force TimeZone=UTC on every connection (app, migrations, replicas, psql), and convert to a named zone only at the edge. timestamp without time zone is fine for things that are not instants (a store's opening hours, a birthday) and a footgun for anything that is.