People are changing the date on their 3DS. The date something is caught is directly dependent on what the 3DS' time settings are.
I changed the date to test this phenomenon, and it does happen. I changed it back and the date on the first Pokemon (a Starly, in this experiment) was dated four years apart from the new one, when in fact I caught them maybe seven minutes apart. I mean, this was in DP, but it can happen in any game.
I had some idiot send me a Kyurem dated Christmas Eve 2007 back in the day when I was on the BW2 GTS.