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+09:00'.
SET TIME ZONE INTERVAL '09:00' HOUR TO MINUTE; SELECT CAST(TIME '08:30:00' AS TIMESTAMP(0) WITH TIME ZONE); SELECT CAST(TIME '08:30:00' AS TIMESTAMP(0) WITH TIME ZONE 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' HOUR 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' obtained after the preceding adjustment. Then this timestamp '2008-05-14 08:30:00' at time zone displacement, INTERVAL '09:00' HOUR TO MINUTE, is adjusted to UTC so that the CAST result is '2008-05-13 23:30:00' at UTC with time zone displacement, INTERVAL '09:00' HOUR TO MINUTE.
The result value of the CAST at UTC is adjusted to time zone displacement, INTERVAL '09:00' HOUR TO MINUTE, so the result of the SELECT statements is: TIMESTAMP '2008-05-14 08:30:00+09:00'.