Unable to create Function Based Index on TIMESTAMP WITH TIMEZONE Column

Guru's

I have TIMESTAMP WITH TIMEZONE Precision 6 column(EX: 29-JAN-15 03.50.01.000000000 PM) in database and all my BI queries query that column as CALLDATE BETWEEN TO_DATE('2015-02-02 00:00:00' , 'YYYY-MM-DD HH24:MI:SS') and TO_DATE('2015-03-02 00:00:00' , 'YYYY-MM-DD HH24:MI:SS') , I am trying to create FBI on CALLDATE as follows

CREATE INDEX FBI ON TABLE1(TO_DATE(CALLDATE , 'YYYY-MM-DD HH24:MI:SS'));

I get the following error, I understand that I am trying to convert non-numeric to numeric character, Is there any way I can create FBI.

SQL Error: ORA-01858: a non-numeric character was found where a numeric was expected