Oracle SYS_EXTRACT_UTC trying to understand - oracle

Question

Here is one of our query in the database, i am trying to understand some UTC converstion in our query.
Can some one briefly explain, what the below query is doing?
SELECT
CAST (SYS_EXTRACT_UTC (CAST ( (BEGIN_DATE - (3 / 24)) AS TIMESTAMP)) AS DATE) BEGIN_DATE
FROM offer o
WHERE mask = 'OK'
AFTER CONVERSION OUTPUT : 06-SEP-11 04:00:00
SELECT
BEGIN_DATE
FROM offer o
WHERE mask = 'OK'
BEFORE CONVERSION OUTPUT : 06-SEP-11 00:00:00

Answer 1

SYS_EXTRACT_UTC extracts the UTC (Coordinated Universal Time—formerly Greenwich Mean Time) from a datetime value with time zone offset or time zone region name.
For example,
SQL> SELECT SYS_EXTRACT_UTC(TIMESTAMP '2014-11-05 12:00:00.00 -08:00') as dt
2 FROM DUAL;
DT
---------------------------------------------------------------------------
05-NOV-14 08.00.00.000000000 PM
SQL>
In your case, your timezone is -08:00. So, -3/24 is used.
For example, my timezone is +05:30. The current time is,
SQL> select to_char(sysdate, 'mm/dd/yyyy hh:mi:ss am') dt from dual;
DT
----------------------
11/04/2014 01:12:21 pm
SQL>
The UTC time equivalent is,
SQL> SELECT SYS_EXTRACT_UTC(TIMESTAMP '2014-11-05 01:12:21.00 +05:30') as dt
2 FROM DUAL;
DT
---------------------------------------------------------------------------
04-NOV-14 07.42.21.000000000 PM
SQL>