PL/SQL :: Error When Subtracting 2 Dates In Package Using Toad?

I am having a problem when calculating dates in a package usiong Toad. I have my function : 

FUNCTION PORTFOLIO_MEMO_RPT_Sql( schema_name IN  VARCHAR2  , select_business_date IN  DATE ) RETURN VARCHAR2.....SELECT DISTINCT       Case     when NVL(TO_DATE(''09/24/2013'',''MM/DD/YYYY'') - DECODE(mn.Active, 1, mn.DueDate, sc.DueDate ),0) = 0 then ''0Days''     when NVL(''' || select_business_date  || '''   - DECODE(mn.Active, 1, mn.DueDate, sc.DueDate ),0) = 0

then ..... when I give it my variable from outside , it is not working......but when I use a date inside, it works. I call my function from outside like :

SELECT * FROM  TABLE ( FI_rpt.Fi_Rpt_Pkg_loan.PORTFOLIO_MEMO_RPT (sys_context('userenv','session_user'),TO_DATE('09/24/2013','MM/DD/YYYY')) )