One of our Support Specialists just came across this bug in Sage 100 v2013pu4, and confirmed it with Sage. (I don't believe this has been previously posted in SC.)
It occurs on AR and AP Invoices. If the Invoice date is Jan 30th or Jan 31st and you are using a 30 day Terms Code value with the Day of the Month box selected (see screen-shot), Sage 100 will calculate the Due Date as 3/30 instead of the beginning of March. The workaround is to hard-code the Due Date field when entering the invoice. If the Day of the Month box is not selected, the Due Date is calculated correctly. Also, if the invoice is entered with any other date in the year with the box selected, the Due Date is calculated properly.