Ars Technica may have found the most logical explanation for the Do Not Disturb bug in iOS this year:
Here’s how it works and why it’s throwing DND for a loop. The ISO week numbering system uses the YYYY format for the year instead of the Gregorian calendar’s yyyy. It then looks at which week of the year it is, and then uses a date digit with 1 starting on Monday. So, for example, Tuesday of the 50th week of 2012 would have been 2012-W50-2 in ISO week format.
Now the question is: why is Apple using week numbers for something that is being dealt with on a daily basis?
The conclusion of the article is right. Although Apple is embarrassed by the problem, creating and pushing a fix would take more time than wait for January 7th.