30 Day Trade Credit

I know there is in store credit and such but, I was wanting to be able to limit to 30 days possibly assigned to user groups for ease of use and implementation that would allow selected users/groups 30 days of pre-set levels of credit before payments would be required. Basically for trade customers we assign, they get credit, everyone else has to pay by card.

I know i'm not the only one that is currently looking to do this and from what I have found it is just not possible