How to find tz offset for a particular day (not today)

Question

I have DATE values that need to be adjusted based on the tz and dst offset in effect on the date. I'm trying to figure out how to calculate the value to adjust them by.
tz_offset(tz) returns the timezone offset for the given tz for today. Is there a way to determine the offset for a timezone for another day, so that daylight savings time differences are honored?
Thanks.

Answer 1

Just double checked an alternative method, once your data is a timestamp with timezone, you can use the extract function to get just the parts of it you are interested in e.g.
with sample_data as (select TO_TIMESTAMP_TZ('2007/05/10:06:00:00pm', 'RRRR/MM/DD:HH:MI:SSpm') ts from dual
  union all select TO_TIMESTAMP_TZ('2009/02/22:05:00:00pm', 'RRRR/MM/DD:HH:MI:SSpm') from dual
select EXTRACT(TIMEZONE_ABBR FROM ts) TZ
     , EXTRACT(TIMEZONE_HOUR FROM ts) TZH
     , EXTRACT(TIMEZONE_MINUTE FROM ts) TZM
     , to_char(ts, 'tzh:tzm') x
     , ts
  from sample_data
TZ         TZH                    TZM                    X      TS           
PDT        -7                     0                      -07:00 10-MAY-07 06.00.00.000000000 PM AMERICA/LOS_ANGELES
PST        -8                     0                      -08:00 22-FEB-09 05.00.00.000000000 PM AMERICA/LOS_ANGELES
2 rows selected