='>=' & Date(AddMonths(Max([MONTH YEAR Calender]),-12), 'MMM-YYYY') & '<=' & Date(AddMonths(Max([MONTH YEAR Calender]), 0), 'MMM-YYYY')
In a text box object to see what date range is this showing and what is the format? Is this giving you the right range and right format?
Second thing to check is if you have other date and time related field where you are making selections? If there are other fields (for instance Month or Year), you will have to add them to your expression so that your expression doesn't filter out based on selection in those date and time related fields