Daylight Savings Time Confusion

Question

Hello,
I am trying to understand how Daylight Savings Time is handled in Oracle Database.
I have read through the "Oracle Database Globalization Support Guide", in particular this part onwards to understand how it works.
However, I have some doubts about the way things work.
I am particularly interested in how the ambiguous date-times are handled.
I have been trying to find the reason as to why same query returns different values when executed through SQL Developer and from SQL*Plus
Following are the details:
In SQL*Plus session
SQL> select * from v$version ;
BANNER
Oracle Database 11g Release 11.2.0.2.0 - 64bit Production
PL/SQL Release 11.2.0.2.0 - Production
CORE    11.2.0.2.0      Production
TNS for Linux: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production
SQL> select sessiontimezone, dbtimezone from dual ;
SESSIONTIMEZONE
DBTIME
+01:00
+00:00
SQL> select to_timestamp('31-MAR-2013 01:00:00','DD-MON-YYYY HH24:MI:SS') from dual ;
TO_TIMESTAMP('31-MAR-201301:00:00','DD-MON-YYYYHH24:MI:SS')
31-MAR-13 01.00.00.000000000
SQL> alter session set time_zone = 'Europe/London' ;
Session altered.
SQL> select to_timestamp('31-MAR-2013 01:00:00','DD-MON-YYYY HH24:MI:SS') from dual ;
TO_TIMESTAMP('31-MAR-201301:00:00','DD-MON-YYYYHH24:MI:SS')
31-MAR-13 01.00.00.000000000
SQL> select to_timestamp('31-MAR-2013 01:00:00','DD-MON-YYYY HH24:MI:SS') at time zone 'Europe/London' from dual ;
select to_timestamp('31-MAR-2013 01:00:00','DD-MON-YYYY HH24:MI:SS') at time zone 'Europe/London' from dual
ERROR at line 1:
ORA-01878: specified field not found in datetime or intervalAs you can see, when the clocks changed on the morning of 31st March, the time between 01:00:00 and 01:59:59 does not exist (as the clocks are advanced by one hour at 01:00:00 AM to 02:00:00 AM). However, the TO_TIMESTAMP function returns the "invalid" date.
But when I execute the same query (i.e. select to_timestamp('31-MAR-2013 01:00:00','DD-MON-YYYY HH24:MI:SS') from dual) against the same database using SQL Developer, I get "31-MAR-2013 02.00.00.000000000" as the result.
SESSIONTIMEZONE     DBTIMEZONE
Europe/London     +00:00Can somebody help me understand this behaviour ?
Thanks in advance.
Edited by: user503699 on Apr 19, 2013 3:28 PM

Answer 1

sorry, i don't explain it very well.
I mean that the same time_zone name can be the same, but the value depend of the time zone table:
For example:
TZNAME                                              TZABBREV
Europe/London                                                    LMT
Europe/London                                                    GMT
Europe/London                                                    BST
Europe/London                                                    BDSTLMT
Local Mean Time. This is the 'natural' mean solar time for the time zone, excluding the equation of time. At the moments where the equation of time is zero, this is the amount of time to be added to UTC. Right then the sun passes the local meridian and is at its highest point in the sky.
This means that the sun is at its highest point at local time:
12 + zonetime + DST - LMT - EquationOfTime
Example: Kaxgar, China; location: 39:29°N, 075:59°E; LMT: 5:03:56; zone: 8; DST: 0; EoT on 2005-07-29: -0:06:28; this gives local time 15:02:32.
Over a year the equation of time can make a difference of between about + and - 18 minutes with the mean LMT time. The LMT time is calculated from the geographic longitude of a local place. Divide the number of degrees in longitude by 15.GMT:
Greenwich Mean Time (GMT) is a time system originally referring to mean solar time at the Royal Observatory in Greenwich, London,BST:
British Summer Time
Time zone offset: UTC + 1 hour
BST is 1 hour ahead of Coordinated Universal Time (UTC)
Note that BST is a daylight saving time/summer time zone. It is generally only used during the summer in the places listed below, during the winter GMT or IST are used instead*BDST i cant find.