The problem is in AX2012 when the Terms of payment has setup for 30 days of the next month. If we create the Invoice journal in a month, which has 31 days (e.g. August), the Due date is calculated OK (30th of September). If  we create the Invoice journal in a month which has 30 days (e.g. September) the Due date is calculated incorrect + 2 months (30th of November), but should be October.

The Workaround you can find in the attached file.