Get date value of parameter and subtract 1 month

Hi

I have a parameter in my report that prompts the user to select a Billing Date and I display that on my report.

However, now I need to get the date from the BillingDate parameter, subtract 1  month and display the month name.

I am having difficulty figuring out how to subtract 1 month from the @BillingDate parameter and display that.

Thanks.