Apple aren't really the kind of company that would come out with details.
It's been suggested that the fix to the last alarm bug has caused this problem. Phones running older OS versions weren't affected.
I was affected on 4.01.
Older as in not iOS4. Like my iPhone 3G running 3.1.3.
Things get lined up again when we passed the date for the clocks to change in America, when everything got changed to the right format.