Pass dynamic Value to SSIS package

I have an SSIS package that has a variable called @RUN_DATE.

I would like to create a sql agent job and uses the set values tab to pass a dynamic parameter to the ssis package.

for example, i would like to always set the variable to the last day of the previous month. I know that If i hardcode the date as 5-31-2013 it works fine, but i have not been able to figure out how to make the value dynamic by using an expression or if this is even possible.

any help would be appreciated.

thanks
Scott