Date formulas

I have been asked to set up a process to auto-create a task when a campaign record is created. And they want the due date of the task to be the first of the month, at least 2 months prior to the campaign start date. 

So if the campaign start date is 2/15/17, then the due date of the task would be 12/1/16. Any ideas on a formula to get me there?

Thanks!