now() is local, as I expect:
testdb=# select now();
2018-06-25 16:41:28.037072-07
And converting to UTC does convert to UTC:
testdb=# select now() at time zone 'utc';
2018-06-25 23:41:23.700795
But converting that timestamp to UTC a second time converts back to local:
testdb=# select (now() at time zone 'utc') at time zone 'utc';
2018-06-25 16:43:03.200762-07
This seems to happen regardless of where the UTC timestamp comes from. Here's the same thing done with a subquery:
testdb=# select (ts at time zone 'utc') from (select now() at time zone 'utc' as ts) as t1;
2018-06-25 16:44:05.219322-07
This seems very wrong to me. But this also seems like something that would have been exercised many, many times in the wild.
I'd expect "converting" a UTC timestamp to UTC would keep it in UTC. Am I missing something?