One of the goals I have for the Challenges is that it's consistent and easy. I also don't want to go back to the old way of the mod staff having to create an entirely new thread for the voting. This was a dreaded and sometimes difficult task. As it is now, all the mod staff need do is add a poll to the Challenge thread. As I stated before, I also don't want the challenge starters to have to figure out an end time for the challenge.
In experimenting with the null poll idea, I remembered that once a poll has been added to a thread, only admins can remove that poll in order for a new one to be added. I don't want to introduce yet another unnecessary step that would further complicate the process. I want all staff/mods to easily be able to transition the challenge thread into a voting thread at the end of the allotted challenge time.
Furthermore, the more I think about the time frame, the less I feel the importance of having a definitive end time right down the absolute second. Most, if not all members can and do complete their submissions without any problems within the designated time frame.
While I completely understand your point/logic, regardless of the challenge's post time or date for that matter, I feel that if they know that the challenge time frame begins on the first day of the month and runs for 16 days, then they know the challenge will end at midnight on the 16th no matter where they are located. I know that this is not a precise method and that's acceptable. If there needs to be any moderation of a late submission, then that will be taken into consideration at the time.