Having trouble with entering expiry date

I could be missing something, but I'm having trouble entering an expiry date to the role. The way it is set up, the user will complete a set of tasks, and then the role will be assigned to them automatically upon completion. (Using the Checklist module) This is a list of tasks they will have to complete annually, during a certain month, so I want the role's automatic expiry date to be the last day of the previous month.

Example: They complete this thing in December 2018, and the role is granted to them until November 30, 2019, at which time they must complete the task again to renew their role. I've tried all sorts of ways to enter the date November 30, 2019 and can't get the formatting right. It keeps referring me to the "strtotime-compatible relative form" page. I've read through that documentation, but I'm not sure what I've missed.

Any help would be appreciated.