Running against POA v8.0 / schema v1.02...

What is the best/right way to mark a "tasklist" (aka "checklist") item "complete"?

At present, I am trying to mark it complete by setting Appointment.checklist.completed = DateTime.Now (i.e. setting the "completed" value to something meaningful), and submitting the modified appointment in a modifyItemRequest() call.

But this doesn't seem to work.

Is there a better way? Or am I missing a step?