The problem is in the class account_payment_term from the file account.py in the account addon. There is a line in the method copute() that says:

next_date += relativedelta(day=line.days2, months=1)

If you change months to 0, it might be wrong when the amount of days in the "days" field results in a smaller day date than the one specified in days2.

I tried to post a link to the module we created to solve it, but it seems I'm not allowed to do so.

 Hi E.m.,

W.r.t your query I have gone through it and tried to replicate it. Few recommendations which I would like to suggest are :-

1) By default in Odoo if we set Day of the Month as positive it gives the day of the next month. And this is what is happening in your case.This scan be done by following Accounting>Configuration>Miscellaneous>Payment Terms

 2) My recommendation would be to set Day of the Month as Zero, if you do so it would be calculate for net days. By doing so I hope you issue might get resolved

3) Also refer to below link for more info on the payment terms

\https://doc.odoo.com/6.1/book/3/3_9/payment_terms/

Hope this helps.

 Cheers!!