Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Do Not Disturb iOS Glitch Now Fixed
iOS 6 did not get off to a good start with the Do Not Disturb feature being disabled on schedule after the feature was developed on the Gregorian calendar which basically meant that the code still thought it was 2012 instead of 2013.
Apple reacted straight away and said the issue would be fixed on the 7th January and they have not let the side down with the feature / glitch being fixed. We can hopefully see this fixed with in iOS 6.1 or iOS 7 which would hopefully stop anything of this sort happening this time next year.