Changing period to post on Enter Bill form

I cannot figure out how to change the posting period when entering a bill if the invoice date is in in a different month than the month I would like to post. There a few different circumstances in which I will need to do this. I have tried changing the override period restrictions for the role/user and attempted to customize the form itself. The period to post on the enter bill form is disabled and will correspond with whatever date I enter for the invoice date. Even though in "customize form" this field is set to "normal" it stays disabled.