The issue is most likely a very simple one, and one that date programmers and heck any programmer can hit VERY easily.
In date formatting you use letters to signify the kind of format you want to use - such as MM for month, or MMM for three letter month name. One common mistake, for example, is using YYYY for year and not yyyy. YYYY gives you the year according to the WEEK the day falls in, while yyyy gives you the calendar year. This bug hit Apple in iPhoto a couple years back and even in iOS as well I think. I'm not saying THIS bug is related to the case of a Y, but it most certainly could be something like that.
And they may have tested it in 2011 against 2012 and it was fine - but broke in 2013 for a different reason.
Calendar/date arithmetic is HARD. There are many idiosyncrasies and edge cases and weird things. Anybody here who thinks they could do better is just dreaming.
I've been working with date math for 12 years now and consider myself an expert at it more so than most any developer I know (and I know a lot) and when I bring new developers in they think they know it all - I mean how hard can it be? 28-31 days in a month, 12 months, 24 hour days, etc... right? Add timezones, non-gregorian calendars, leap year, DST, and on top of it all the different formatting across the world and you've got yourself something that isn't simple.
So give Apple a bit of a break. As another poster put it, at least they didn't lose an entire month in their calendar app (cough cough Google).