Do you think P5 takes place in 2016? It makes sense because of the calendar but I think it's weird why they put 20XX. Maybe because they don't want to reveal yet how many years are P4 and P5 apart?
It’s cemented to be set in 2016 by the cameos of Chie and Rise; Rise’s cameo mentions she’s 20+, while Chie’s cameo mentions she’s still in Police Academy.
Unless Chie was held back for half an eternity, which I doubt, the only year in which this is possible with the calendar we have is 2016.
I’m fairly certain the date is only obscured by the XXs because the game refers to events and people who might potentially resemble real life events or people. There’s a law against that in Japan, so putting a distinct date there could possibly open ATLUS up for a lawsuit. Turning the 16 into a pair of XXs was the best way to avoid that.