Assuming the current date is DATE '2008-05-14' at time zone displacement, INTERVAL '09:00' HOUR TO MINUTE, the following SELECT statements return the result: TIMESTAMP '2008-05-14 08:30:00'.
SET TIME ZONE INTERVAL '09:00' HOUR TO MINUTE; SELECT CAST(TIME '08:30:00' AS TIMESTAMP(0)); SELECT CAST(TIME '08:30:00' AS TIMESTAMP(0) AT LOCAL);
The current session time zone displacement, INTERVAL '09:00' HOUR TO MINUTE, is used to determine the UTC value '23:30:00' of the literal.
For the CAST, the source expression value '23:30:00' at UTC is adjusted to the current session time zone displacement, INTERVAL '09:00' TO MINUTE, to yield '08:30:00'. A timestamp is formed from the current date '2008-05-14' at time zone displacement, INTERVAL '09:00' HOUR TO MINUTE, and the time portion of the source expression value '08:30:00'. Then, this timestamp, '2008-05-14 08:30:00', at time zone displacement, INTERVAL '09:00' HOUR TO MINUTE, is adjusted to UTC so the CAST result is '2008-05-13 23:30:00' at UTC.
The result value of the CAST at UTC is adjusted to the current session time zone displacement, INTERVAL '09:00' HOUR TO MINUTE, so the result of the SELECT statements is: TIMESTAMP '2008-05-14 08:30:00'.