Oracle : How to change the start time of an schedule.

BEGIN
sys.DBMS_SCHEDULER.SET_ATTRIBUTE (
name => '"{USER}"."{SCHEDULE_NAME}"',
attribute => 'START_DATE',
value => '28-NOV-08 12.00.00.000000 AM');
END;
/

If you want to set to another timezone :

BEGIN
sys.DBMS_SCHEDULER.SET_ATTRIBUTE (
name => '"{USER}"."{SCHEDULE_NAME}"',
attribute => 'START_DATE',
value => '28-NOV-08 12.00.00.000000 AM MET');
END;
/

http://www.dba-oracle.com/t_ora_01840_input_value_not_long_enough_for_date_format.htm