How to convert a column timezone from CET to CST in SQL query - sql

Question

Here I am trying to convert date column value from CET to CST.
I tried using NEW_TIME(SYSDATE ,'CET','CST') function, but it's giving an error saying unknown time zone. Issue here is CET is not recognised by oracle as valid timezone.
I tried using "at timezone" approach initially, but it's inserting the timezone name in the column value, which I don't want.

Answer 1

The three-character timezones don't adjust for daylight savings. To do that you need to use the fully specified timezone name. To illustrate, in Canada, there is central standard time, but the province of Saskatechewan does not use daylight savings, so if I want to convert right now to local time in summer where daylight savings is in play, and knowing the correct fully specified timezone names in the DB Timezone file (You can check the list installed in your DB by select * from V$TIMEZONE_NAMES;):
SELECT 'Central' locale
, extract(timezone_abbr from cast(add_months(sysdate,6) as timestamp) AT TIME ZONE 'Canada/Central') tz_abbrv
, CAST(cast(add_months(sysdate,6) as timestamp) AT TIME ZONE 'Canada/Central' as timestamp) local_time from dual
union all
SELECT 'Saskatchewan' locale
, extract(timezone_abbr from cast(add_months(sysdate,6) as timestamp) AT TIME ZONE 'Canada/Saskatchewan') tz_abbrv
, CAST(cast(add_months(sysdate,6) as timestamp) AT TIME ZONE 'Canada/Saskatchewan' as timestamp) local_time from dual;
LOCALE TZ_ABBRV LOCAL_TIME
Central CDT 03/08/2016 9:57:24.000000 AM
Saskatchewan CST 03/08/2016 8:57:24.000000 AM
Otherwise you would need to code when to switch between CST and DST in your calculations - bearing in mind that the rules for when daylight savings starts and ends has changed over time, and may change again in the future.
So respond to your comment about inserts, you need to first ensure that the column is defined to include the time zone (datatype "timestamp with timezone"), and change the CAST from "timestamp" to "timestamp with time zone" to make sure that the zone information is stored:
e.g.)
create table mbt (stz timestamp with time zone)
insert into mbt values (CAST(cast(add_months(sysdate,6) as timestamp) AT TIME ZONE 'Indian/Maldives' as timestamp with time zone) )
insert into mbt values (CAST(cast(add_months(sysdate,6) as timestamp) AT TIME ZONE 'Canada/Eastern' as timestamp with time zone) )
commit;
select * from mbt;
STZ
05/08/2016 8:06:49.000000 PM +05:00
05/08/2016 11:06:50.000000 AM -04:00
If you don't include the WITH TIME ZONE you should still get the changed value, but you won't be able to easily translate from local time to a different timezone as the values will be assumed to be in the server timezone:
drop table mbt;
create table mbt (stz timestamp);
insert into mbt values (CAST(cast(add_months(sysdate,6) as timestamp) AT TIME ZONE 'Indian/Maldives' as timestamp) );
insert into mbt values (CAST(cast(add_months(sysdate,6) as timestamp) AT TIME ZONE 'Canada/Eastern' as timestamp ) );
commit;
select * from mbt;
STZ
-------------------------------
05-AUG-16 08.13.16.000000 PM
05-AUG-16 11.13.16.000000 AM

Answer 2

To remove timezone you can use cast(.... as Timesamp).
Check my example.
select cast( current_timestamp at time zone 'CST' as timestamp), cast( current_timestamp at time zone 'CST' as timestamp with time zone) from dual;