Assume that the current date is DATE '2008-05-14' at time zone displacement, INTERVAL '9:00' HOUR TO MINUTE, but the current date is DATE '2008-05-13' at time zone displacement, INTERVAL '04:00' HOUR TO MINUTE. The following SELECT statement returns the result: TIMESTAMP '2008-05-13 13:30:00'.
SET TIME ZONE INTERVAL '09:00' HOUR TO MINUTE; SELECT CAST(TIME '08:30:00+04:00' AS TIMESTAMP(0) AT SOURCE TIME ZONE);
The time zone displacement, INTERVAL '04:00' HOUR TO MINUTE, in the literal is used to determine the UTC value '04:30:00' and time zone displacement, INTERVAL '04:00' HOUR TO MINUTE, of the literal. For the CAST, the source expression value '04:30:00' at UTC is adjusted to the time zone displacement of the source, INTERVAL '04:00' HOUR TO MINUTE, to yield '08:30:00'.
A timestamp is formed from the current date '2008-05-13' at time zone displacement, INTERVAL '04:00' HOUR TO MINUTE, and the time portion of the source expression value '08:30:00' obtained after the time zone displacement adjustment. Then this timestamp '2008-05-13 08:30:00' at time zone displacement, INTERVAL '04:00' HOUR TO MINUTE, is adjusted to UTC so that the CAST result is '2008-05-13 04:30:00' 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 statement is: TIMESTAMP '2008-05-13 13:30:00'.