I'm sorry but I'm not following your logic here. Why can't the day occur on 28? It is calculating the calendar week (i.e. the week aligned with the calendar.) When I look at the last week in January, 2007, I see that the beginning of the week according to the Gregorian calendar (and assuming Sunday is the first day) falls on the 28th. If you check this under Tiger, you get {28, 7}. That means the calendar date starts on the 28th and lasts 7 days.

Leopard reports {1, 31}. Now, even if you take that 1 to be the first of the next month, 31 still doesn't make sense under your logic because the next month is February and it only has 28 days. But all that is moot as the week does not start on the first for either Jan or Feb of 2007. In addition, back to the original bug I cited, there is no good reason why the week, which was accurately reported as being 7 days long in the other weeks (and in every week on Tiger) is reported as being 31 days. Of what possible use is that to the developer? It seems like you are trying to argue the "logic" of the bug. I just don't see how you can argue this as being logical. It may be an explanation of why it's happening, but that doesn't mean it's correct.