oracle 中处理时间（时间如：20-DEC-17 02.30.30.000000000 PM

SELECT TO_CHAR(TO_TIMESTAMP(SIGN_TIME,
'DD-MON-YY HH.Mi.SS.FF PM'),'YYYY-MM-DD HH24 mi:ss')
FROM T_GMS_REALTIME_STRATEGY_CUST_D
报错：
[SQL]SELECT TO_CHAR(TO_TIMESTAMP(SIGN_TIME,
'DD-MON-YY HH.Mi.SS.FF PM'),'YYYY-MM-DD HH24 mi:ss')
FROM T_GMS_REALTIME_STRATEGY_CUST_D
[Err] ORA-01843: not a valid month