首先通过dbms_flashback.get_system_change_number 可以获得系统当前的SCN值：

 SQL> col scn for 9999999999999
 
 SQL> select dbms_flashback.get_system_change_number scn from dual; 
  
SCN
        5223711
 再通过SCN找出其时间
     select scn_to_timestamp(5223711) scn from dual;