How to alter database hour and date in Oracle?
ALTER DATABASE

set_time_zone_clause::=




  set_time_zone_clause
  
  This clause has the same semantics in CREATE DATABASE and ALTER
  DATABASE statements. When used in with ALTER DATABASE, this clause
  resets the time zone of the database. To determine the time zone of
  the database, query the built-in function DBTIMEZONE. After setting or
  changing the time zone with this clause, you must restart the database
  for the new time zone to take effect.
  
  Oracle Database normalizes all new TIMESTAMP WITH LOCAL TIME ZONE data
  to the time zone of the database when the data is stored on
  disk.Oracle Database does not automatically update existing data in
  the database to the new time zone. Therefore, you cannot reset the
  database time zone if there is any TIMESTAMP WITH LOCAL TIME ZONE data
  in the database. You must first delete or export the TIMESTAMP WITH
  LOCAL TIME ZONE data and then reset the database time zone. For this
  reason, Oracle does not encourage you to change the time zone of a
  database that contains data.
  
  For a full description of this clause, refer to set_time_zone_clause
  in the documentation on CREATE DATABASE.


SQL&gt; select dbtimezone from dual;

DBTIME
------
+02:00

SQL&gt; ALTER DATABASE SET TIME_ZONE = '-05:00';

Database altered.

SQL&gt; select dbtimezone from dual;

DBTIME
------
+02:00

SQL&gt; startup force
ORACLE instance started.

Total System Global Area  536868920 bytes
Fixed Size                  8898616 bytes
Variable Size             226492416 bytes
Database Buffers          293601280 bytes
Redo Buffers                7876608 bytes
Database mounted.
Database opened.
SQL&gt; select dbtimezone from dual;

DBTIME
------
-05:00