BUG #19053: Inconsistent arithmetic regarding TIMESTAMPTZ and INTERVAL

The following bug has been logged on the website:

Bug reference:      19053
Logged by:          Jan Behrens
Email address:      jbe-mlist@magnetkern.de
PostgreSQL version: 17.5
Operating system:   FreeBSD
Description:

Consider the following calculations:

jbe=# SET TIME ZONE 'Europe/Berlin';
SET
jbe=# SELECT TIMESTAMPTZ '2025-10-27' - TIMESTAMPTZ '2025-10-26';
    ?column?
----------------
 1 day 01:00:00
(1 row)

jbe=# SELECT TIMESTAMPTZ '2025-10-26' + INTERVAL '1 day 01:00:00'; -- the
following result is surprising, as it is not midnight
        ?column?
------------------------
 2025-10-27 01:00:00+01
(1 row)

jbe=# SELECT TIMESTAMPTZ '2025-03-31' - TIMESTAMPTZ '2025-03-30';
 ?column?
----------
 23:00:00
(1 row)

jbe=# SELECT TIMESTAMPTZ '2025-03-30' + INTERVAL '23:00:00'; -- inconsistent
with the previous addition above, as it is midnight
        ?column?
------------------------
 2025-03-31 00:00:00+02
(1 row)

jbe=# SELECT TIMESTAMPTZ '2025-04-01' - TIMESTAMPTZ '2025-03-30';
    ?column?
----------------
 1 day 23:00:00
(1 row)

jbe=# SELECT TIMESTAMPTZ '2025-03-30' + INTERVAL '1 day 23:00:00'; -- here,
the result isn't midnight again
        ?column?
------------------------
 2025-03-31 23:00:00+02
(1 row)

Or, some of these operations written in a single expression:

jbe=# SELECT TIMESTAMPTZ '2025-03-30' + (TIMESTAMPTZ '2025-03-31' -
TIMESTAMPTZ '2025-03-30');
        ?column?
------------------------
 2025-03-31 00:00:00+02
(1 row)

jbe=# SELECT TIMESTAMPTZ '2025-03-30' + (TIMESTAMPTZ '2025-04-01' -
TIMESTAMPTZ '2025-03-30');
        ?column?
------------------------
 2025-03-31 23:00:00+02
(1 row)

Note that there is no time zone change in between 2025-03-31 and 2025-04-01,
yet the time of the previous two calculations is off by one hour.