Minisode apparently set 24 hours after Bad Night.
In the TARDIS, Amy pinions down the Doctor on what's become of her life history. (or should that be her life herstory?)
For my money the explanation that Amy and Rory both now have two contradictory sets of memories really ought to have been covered in The Big Bang, not eighteen months later if you're willing to pay extra for the DVD that features this minisode.
Again throwing his own rule about not crossing personal timelines to the wind, the Doctor takes Amy back into her youth (alright even further back into her youth) and messes with her head even more, purely to prove a point you understand.
Like a scientific experiment, it's important to observe what happens to Amy's memory as the Doctor takes her back to that childhood day. Before the journey back in time, she says that she was alone that day. During the journey back in time, she still says that she was alone that day. However, as soon as the TARDIS materialises, her memory updates with the event of her having met her older self, despite it not having actually taken place yet for either Amy.
Conclusion: History seems to update at the moment when the TARDIS first enters and changes the timestream. From this moment on, all of the Doctor's and Amy's imminent actions in this time and place are already a part of history, although they have yet to cause them. Presumably the effects of their next visit will not be a part of the universe's history until they have first landed there either.
Nobody in the series has clearly explained how time-travel works in Doctor-Who-Land yet. They tend to just hide behind the words "It doesn't work like that."
It's great to see author Steven Moffat expounding his vision of how time-travel works, but again, I really needed to be informed of his system before trying to follow something like A Christmas Carol, or it can just look like a load of contradictions.