If you still have the stub (or can access a copy of the check) take a look at it.
My employees work Monday through Sunday and receive their checks on Wednesday but the checks are dated on that Sunday.
So, for example, if an employee worked December 28, 29, 30 of 2015, she’d get a check dated January 2, 2016. She could quit on December 31st 2015 and never actually work for me this year but still receive a 2016 W-2*.
2014 ended in the middle of a work week. That’s probably what happened. He probably worked the beginning of the week (2014) and the check was dated for the end of the pay period (2015).
It’s a huge PITA to do an unscheduled payroll and there’s not really any advantage to doing it so as far as paychecks are concerned, it’s business as usual, just like any other week.
*This is why I always tell people you shouldn’t run and do your taxes the very second you can. I’ve had friends do their taxes as soon as they get their W-2 only to have to have them amended when they get another w-2 or 1099 from a place they forget they worked for a few weeks or from a savings account they forgot about. It’s one thing if you KNOW you only worked at one job, but if you hop around, it’s safer to wait until a week or two after January 31st and a bit after Feb 15th if you might get 1099s.