Can you change the help text for `extended_due` to read something like:

```
+    extended_due = Date(
 +        help="Date that this problem is due by for a particular student. This "
 +             "can be set by an instructor, and will override the global due date "
+              "if it is set to a date that is later than the global due date."
```

Feel free to tweak my exact language there.