On New Year's day millions of iOS 6 users began to notice they weren't getting their self-selected window of daily disturbance. Do Not Disturb is a feature in iOS 6 to disable notifications. The scheduled feature failed to enable itself on January 1, 2013 leading to Apple support being inundated with users reporting that Do Not Disturb stopped working correctly. Apple soon posted a support document claiming that the problem would fix itself after January 7.

The reason this bug occurred is because of calendaring code used  by Apple in iOS 6. Apple uses ISO week numbering in the code which led to the bug and here's reportedly why it happened:

"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.

The problem comes in when January 1 of the New Year ends up falling on a date that doesn't get along well with the ISO week format. The first day of 2013 started on a Tuesday, whereas  the ISO standard expects the first week of the year to start on "the Monday that contains the first Thursday in January." In this case, that would be January 7, 2013. This is a common pitfall that happens at the beginning of the New Year and lines up with the time when the bug will go away."

Apple decided to let the bug fix itself rather than issuing an iOS software update specifically to target the bug. Do Not Disturb is now back to disturbing.