shared business logic is it a model or should i place in controller

[eluser]Michael Wales[/eluser]
I would definitely take something like that and make it a method within my invoices model.

The basic rule of thumb is:
recurring function called from controller = model
recurring function called from view = helper

In your case, something like passing the invoice ID to an is_overdue() function within the model that returns TRUE/FALSE would work perfectly.