convert seperate date and time strings into a single date/time field

[esanray]

My model contain sch_date in a query item & sch_time in another query item.one is date datatype another is numeric data type.

sch date      sch time
18-NOV-11 0620
18-NOV-11 1705
18-NOV-11 2330
18-NOV-11 1500

Report Requre to display the value in timestamp format so i filter it from system time.
i write the following Query but its not working

TO_CHAR(TO_DATE(TO_CHAR(WOS.FLT_OPR.SCH_DATE ,'DDMONYY')||''||LPAD(WOS.FLT_OPR.SCH_TIME ,4,0),'DDMONYYHH24MI')