Wait a Minute...
They say (with certainty) that there was at time in 2012 when three infinity stones were definitely in New York.
But this is wrong.
Only two were there. They're simply making the assumption that the time stone is there because Dr Strange happened to live in New York. But for all we know, he didn't move to New York until 2013. For all we in know, he didn't receive the time stone until 2014. Etc etc...
Which is exactly what we discover in the film -- he didn't have it.
Thus, it becomes slightly convenient that Tilda Swinton just happens to be there... waiting at Dr Strange's apartment (before she ever met him) just as the Hulk shows up.