Need SSIS vaiable value = Last day of privouse month in date formate (ex. 10/31/2012)

Hi All

I am using SSIS2008R2.

I need my variable value like previouse month last day using expressinon and format should be like this 10/31/2012

so please let me know the expression and which datatype i have to use for that variable.

Thanks,

Mark