Just a thought, but knowing that your "time isn't up" isn't the same thing as knowing the exact time it will be up.
What the demons said might be translated as "we know that one day our freedom will be taken away, but we also know that day isn't 'today'."
Side note: Looks like "time was up" for these particular demons...as least as it pertained to their possession of this particular man.
No disagreement with you at all.
but I still find the rest of the account interesting too.