Fantastic, 2012 I can understand but I don't know the idea that it was set in 2021 came from. And since Rory and Amy died before 2012 their can't be a version of them from 2020 standiing on a hill in 2010.
ETA: I suppose the argument could be made that the Doctor took Amy and Rory from 2010 and put them on the hll to be able to wave to themselves, although that really makes no sense at least to me. But it also makes no sense for the Doctor to take the 2020 versions of Amy and Rory to Central Park in 2012. I guess it's just better not to think about it too much.
No one is saying it was set in 2021, just that Amy and Rory were from 2021 and the Doctor took them to 2012, which like you say makes no sense.
And really the whole dating thing makes no sense at all anymore. 2021 is conjecture based on admittedly flimsy evidence, but if you take into account everyhting that has been established in seasons 5 and 6, Amy and Rory can be from no earlier than 2016 in order to accomodate all the time jumping that has been going on.
Really, Doctor Who has never been too consistent with dating modern day stories. The only time any real effort was put into doing so was during the RTD era, and they still screwed things up. Still, I think S7 may be as sloppy as UNIT Dating.