I'm really concerned about Dave's point, and as an instructor I would expect that if I give a few students 2 days extension, then changed the global due dates to 7 days later, that the individual students would get the latest, most generous date. 

@pdpinch you say "the name of the feature the emphasis is on idd and not extension", but the code disagrees with that assertion. Everywhere throughout this code the feature is referred to as "extension", "extended_due", etc. Even the docstring for the field indicates extension:

```
+    extended_due = Date(
 +        help="Date that this problem is due by for a particular student. This "
 +             "may differ from the global due date if an instructor has granted "
 +             "an extension to the student.",
```

With all that in mind, I was actually really surprised that you don't just take whatever date is latest. I vote for erring on the side of benefitting the student, unless you go through this entire pull request and change the name of everything and all the docstrings to something that much more clearly indicates "individual due date" and removes all references to the word "extension". I'm open to further discussion but I am concerned about this.