A task that is overdue can't be dragged to a future hour on the same day it was due.
You can try to drag and drop the task from the upper calendar view part to the calendar, with no effect.

Reproducible: Always

Steps to Reproduce:
1. Let's say we are the 2012-08-07 at 11h00
2. Create a task, unset starting time and set the expiration date to the 2012-08-07 at 15h00
3. Go drink something then come back at 15h01
4. Change of week then come back to this week (to update the calendar view, btw, this could be automatic !)
5. Now you task should be displayed in the upper part of the calendar view, as overdue
6. Try to drag and drop this task to 17h00 the same day : it's impossible

7. If you try once more to drag and drop the task to 17h00 the same day, it's still impossible, but..
8. if you try to drag and drop the task to 19h00 the same day, then it's possible.