ORACLE SQL: add time to a timestamp with timezone - sql

Question

I need to add seconds and substract variables with type TIMESTAMP WITH TIMEZONE, however, to my understanding, adding numbers to such data causes the information about the timezone to be lost, perhaps because it's converted to a DATE type
That is:
SELECT FROM_TZ(
TO_TIMESTAMP(
TO_DATE('03/09/2012 2:30:30','DD/MM/YYYY HH:MI:SS')
)
, 'America/Chicago')
FROM DUAL;
Gives:
03/09/2012 00:00:00, -05:00
Then
SELECT FROM_TZ(
TO_TIMESTAMP(
TO_DATE('03/09/2012 2:30:30','DD/MM/YYYY HH:MI:SS')
)
, 'America/Chicago') + 1/24 -- add 1 hour
FROM DUAL;
Gives
03/09/2012 01:00:00
and loses the timezone information.
But
SELECT FROM_TZ(
TO_TIMESTAMP(
TO_DATE('03/09/2012 2:30:30','DD/MM/YYYY HH:MI:SS'))
, 'America/Chicago') + INTERVAL '1' hour
FROM DUAL;
Correctly gives
03/09/2012 01:00:00,000000000 -05:00
However the INTERVAL.. syntax expect a char constant, so I can't use that with variables.
How can I perform that kind of arithmetic with TIMESTAMP WITH TIME ZONE datatype while retaining timezone information?
TIA

Answer 1

You can use the NUMTODSINTERVAL function to convert a number to an interval.
SELECT FROM_TZ( TIMESTAMP '2012-10-08 00:00:00','-5:00')
+ NUMTODSINTERVAL(1,'HOUR')
FROM dual;