Data Type conversion from VARCHAR2 to TIMESTAMP with time zone on column

I have a source database that has a table with column start_time defined as VARCHAR2.  My destination database has the table column start_time defined as TIMESTAMP with time zone.

I have created the following function. (not sure if it is needed). Can and how do I setup streams to do this type of conversion during the apply process?

Function change_date(date_in VARCHAR2)

begin

   return to_timestamp_tz(date_in, 'fmMMfm/fmDDfm/YYYY fmHH12fm:MI:SS.FF AM TZH:TZM');

end;