The setup for charge date and due date field should be First Day, Last Day, specific date (date can be entered) of the current or next month instead of a list of numbers from 1st to 28th. This applies to all fees such as miscellaneous, special, payment plans etc.
For the date field settings, can a calendar be embedded instead, like in Add payments, charges etc.?
Can you check the Posting procedures under Miscellaneous Fees for the "Each month on..." option? There's no "Last day" selection in the list.