ora-01847 day of month must be between 1 and lastday of month

 sql = "Update Table1 set , column1 = to_date('" & 

ExpensereportReceiveddate.Date & "','mm/dd/yyyy'),  column2 = to_date('" & 

Expensereportacctng.Date & "','mm/dd/yyyy'),  where emp_NO=" & emp_no


When i tried to debug it, Assuming that the user didnt eneter any thing in the date fields..This is the sql it returned....

 
Update travel_auth set  column1 = to_date('12:00:00 AM','mm/dd/yyyy'),  

column2 = to_date('12:00:00 AM','mm/dd/yyyy') where

emp_NO=191


 ExpensereportReceiveddate is declared As Date

How can i solve this.....