If a student joins a class after an assignment due date, why do I have to change the due date of the assignment? Why doesn't khan academy just not be stupid about assigning the work? It seems like a stupid problem: it can't save an edit to a past due assignment, well why not? It seems like they made more work for the programmers to stop an edit to a past due assignment rather than just allowing it.
Other wise, changing the due date will let my other students think that they can submit late work for full credit (which I do not want)