How could I autofill the Expiration Date (override is on) on a node to a relative date?

I currently have a View set up that shows all nodes of a certain content type that were created in the current month. After the current month is over, the nodes are archived in another View. The issue is that any registered user can create this type of node at any time during the current month, so the Expiration Settings "Disable voting after this period" options will not work for this situation. In other words, a node that is created on 3/12 needs to expire at the same time that one created on 3/1 should.

If I enable the override-per-node option, registered users would have to enter the correct end date, which I do not want to allow. That's why I was thinking about autofilling the date and hiding/disabling it to prevent users from screwing with it.