Receivables and O/E

Terms - We are giving Customers terms as follows:

Orders dated from 1st to 15th
Due: End of Month (i.e. 30th or 31st, ignoring February)

Orders dated from 16th to end of month
Due: 15th of next month


We are unable to figure out a single Terms Code that will handle this.
Any suggestions how to best make this work with the least amount of user-input?