Help Extracting Date

I am working with a table that has one of its columns setup to contain multiple pieces of data. For example, it contains username;entered_date;code1;code2;action_date;etc.... Everything is separated by semicolons. I need to get the data out and in separate columns which hasn't been a problem except for the 5th piece, action_date. I didn't set this table up nor do I have any way of changing how the data is entered; it is a little strange but this is what I have to work with. The code I'm working with to get the action_date is the following:

DECODE(substr(mdt.mdta,instr(mdt.mdta,';',1,4)+1,(instr(mdt.mdta,';',1,5))-(instr(mdt.mdta,';',1,4))-1),
'_', TO_DATE(null),

TO_DATE(substr(mdt.mdta,instr(mdt.mdta,';',1,4)+1,(instr(mdt.mdta,';',1,5))-(instr(mdt.mdta,';',1,4))-1),
'ddMonyyyy')) ActionDate

The problem is, I am getting error message ORA-01847: day of month must be between 1 and last day of month. The date format I use in the TO_DATE function is correct. While every row does have something in this column (always the username and entered_date) there isn't always an action_date. Sometimes it is blank or sometimes the user has entered '_' (not sure why but not much I can do about it). How can I rewrite this code so that it pulls and formats the date when it is present and ignores the rest?

Thank you in advance for your help!
Grant Received on Wed Jan 03 2007 - 16:06:40 CST