Originally Posted by AcerBen:
“I think I understand the concept of "personal timelines" and all that, but how is it that the world can end in 1599 if they're living in 2007?
Not really enjoying this episode much. My favourite ones are set in the present.”
That's why they gave the Back to the Future reference to explain it a bit... the 'witches' weren't in the original history, but have entered it as has the Doctor. They're now there trying to bring about the end of the world... if they manage it then 2007 never exists and Martha would fade away.
I actually prefer the ones that aren't set in present day - whether that's in the future or the past.