Oracle SQL HELP with convert GMT to EST and DST and Date offset

Question

Hi, I have a query that does not seem to work trying to convert a date field that is in GMT to est and using extract(timezone_hour FROM TO_TIMESTAMP_TZ as an offsetr
HEre is my sql
dtl.start_dt_gmt + (extract(timezone_hour FROM TO_TIMESTAMP_TZ( dtl.start_dt_gmt,'DD-MON-YYYY HH24:MI:SS TZH:TZM'))/24 ) START_DT_Local
If the date (dtl.start_dt_gmt) is may 1 and gmt starts at 04:00 AM , the extract offset produces -4
However, if the date (dtl.start_dt_gmt) is Feb 1 which begins at 05:00 AM GMT, the date offset still gives 04. What am i doing wrong? Any help would be appreciated. Thanks.
Saul

Answer 1

If your data is not associated with timezone then you'll have to use something like
case when dt between A and B then dt-1/24 else dt end; <-- This will give you 1 hour back of EDT. So, as far as concern at database level, it is nothing to do at db level, because db is used by application, so you need to code in the app.
Oracle never actually changes a TIMEZONE column value when you set your system to be on daylight savings time. There are several built-in DST DATE conversion functions for changing to daylight savings time:
current_date
current_timestamp
localtimestamp
dbtimezone
sessiontimezone
extract
from_tz
to_timestamp
to_timestamp_tz
to_yminterval tz_offset
http://dba-oracle.com/t_oracle_daylight_saving_time_dst_date_conversion.htm
Regards
Girish Sharma