SYSTIMESTAMP and PST again

Question

In a previous question I proposed the following function to determine the current time as represeted in PST:
CREATE OR REPLACE FUNCTION PST_SYSDATE RETURN DATE IS
  th NUMBER;
  tm NUMBER;
  now TIMESTAMP(6) WITH TIME ZONE;
  d  VARCHAR2(200);
BEGIN
  now := SYSTIMESTAMP;
  SELECT
    TO_CHAR(now, 'YYYY-MM-DD HH24:MI:SS'),
    EXTRACT(timezone_hour FROM now) TH,
    EXTRACT(timezone_minute FROM now) TM
  INTO d, th, tm
  FROM DUAL;
  RETURN TO_DATE(d, 'YYYY-MM-DD HH24:MI:SS') - ((th+tm/60+8)/24); 
END PST_SYSDATE;
/ I got some helpful replies which suggested that I use the expression:
CAST( SYSTIMESTAMP AT TIME ZONE TZ_OFFSET('PST') AS DATE)However, my function is not agreeing with this expression:
SQL>  select SYSDATE, PST_SYSDATE,  CAST(SYSTIMESTAMP AT TIME ZONE TZ_OFFSET('PST') AS DATE) PST_DATE from dual;
SYSDATE             PST_SYSDATE         PST_DATE
04/09/2009 11:18:05 04/09/2009 08:18:05 04/09/2009 09:18:06Note that I am in the Central Timezone and we are currently using CDT. 11 AM CDT is 8 AM PST not 9 AM PST.
So is there another expression which can do the same thing as my PST_SYSDATE function?
Thanks!

Answer 1

Ah - this is the problem: when I say 'PST' I mean GMT-8 regardless of the time of year.
I am in the Central time zone and in a region which observes DST. Therefore, right now my offset from GMT is -5, and when reporting times one would normally use CDT to indicate that daylight time is in effect. For instance, my current time is approx. 1:11 PM CDT.
In places where daylight time is not observed (like Ontario), their offset from GMT is -6, and their local time is 12:11 PM CST.
When using the expression AT TIME ZONE TZ_OFFSET('PST'), Oracle seems to be only subtracting two hours when I think it should be subtracting three hours. Perhaps it is a feature. In any case, it is not the behavior I want.
My function does take into account if the database clock uses daylight saving time. Note that SYSTIMESTAMP has a timezone modifier of -5. This is used to determine the time in GMT and then 8 hours is subtracted to obtain the time in PST.
If I run SYSTIMESTAMP during the winter, it will report a timezone offset of -6 (for CST). Again, my function would produce the correct result.
So - I guess the lesson here is that AT TIME ZONE TZ_OFFSET('PST') is not the same as AT TIME ZONE TZ_OFFSET('GMT-8') (if this latter expression were legal in Oracle).
Edited by: user10936714 on Apr 9, 2009 11:24 AM