Trying to retrieve the UTC time-stamp in Oracle, does using "from_tz" cause an issue? - oracle

Question

Will using the following query to retrieve the UTC timestamp from an oracle database cause an issue? I do not want to alter the database timezone parameter in order to retrieve the correct date. I do not want to do alter database set time_zone.
My query at the moment is:
select from_tz(CAST (sys_extract_utc(systimestamp) AS TIMESTAMP), '+00:00') from dual;
I would like to know if this query will result in the correct UTC date in all circumstances regardless of the EST/EDT status.

Answer 1

I don't see anything wrong with your query. Note that if you want to work in UTC for your session, you could simply:
ALTER SESSION SET TIME_ZONE = '0:00';
select CURRENT_TIMESTAMP from dual;
output
11/1/2016 5:48:55.115282 PM +00:00
That would change your current_timestamp (and localtimestamp) for your entire session.

Answer 2

Your query is just setting timezone but not converting time. If that's what you were looking for it is ok. Is local time is 3AM you will return 3AM UTC.
I think you're looking for that query:
select cast(sysdate as timestamp) at time zone 'UTC' from dual;
And the apply
select from_tz(cast(systimestamp at time zone 'UTC' as timestamp), '+0:00') from dual;

Answer 3

Some general notes:
SYSTIMESTAMP and SYSDATE are given in time zone of your database server's operating system. Thus changing database time zone (i.e. DBTIMEZONE) does not change anything.
CAST (sys_extract_utc(systimestamp) AS TIMESTAMP), resp. cast(systimestamp at time zone 'UTC' as timestamp) have a problem. You convert your timestamp to UTC but by CAST(... AS TIMESTAMP) you remove any time zone information from that value. If you like to do any further conversion (e.g. again to TIMESTAMP WITH TIME ZONE value) then your input UTC value is considered to be a SESSIONTIMEZONE value.
from_tz(CAST (sys_extract_utc(systimestamp) AS TIMESTAMP), '+00:00') does following:
Get current time in time zone of your database server's operating system, include time zone information.
Convert this time to UTC, cut time zone information
Append time zone information (+00:00) to this value
Correct output but redundant conversion
cast(sysdate as timestamp) at time zone 'UTC' does following:
Get current time in time zone of your database server's operating system, without any time zone information.
Cast to TIMESTAMP (basically no effect at all)
Consider this value as time in your local session time zone (SESSIONTIMEZONE) and convert to UTC.
Correct output only if time zone of your database server's operating system is the same as your local session time zone, otherwise you get wrong result.
from_tz(cast(systimestamp at time zone 'UTC' as timestamp), '+0:00') does following:
Get current time in time zone of your database server's operating system, include time zone information.
Convert this time to UTC
Cut time zone information
Append time zone information (+00:00) to this value
Correct output but redundant conversion
from_tz(cast(systimestamp as timestamp), '+0:00') does following:
Get current time in time zone of your database server's operating system, include time zone information.
Cut time zone information
Append time zone information (+00:00) to this value
Correct output only if time zone of your database server's operating system is UTC.