Hours 1-8: Work, which, I suppose could be argued, starts the day already with the accomplishment of doing something and earning money. (Perhaps I am actually off the hook for the day.)
Hours 8-9: Travel, which can be the most productive hour of the day, or the least, depending on your mode of transport and your tolerance for noise and crowds.
Hours 9-11: Sleep, a perhaps necessary pause, that either enables the rest of the day (if you wake up refreshed) or obliterates it (if you wake up fuzzy in the head).
Hours 11-?: Okay, what's it gonna be--a bucket of things done, or a stretch that ends with wondering where the day went? A time span the rests on the laurels of those first few hours, or a set of minutes that defines the day?
It could be argued that the first few hours define a day, but with so many hours left, some days simply can't be judged until hour 12, or hour 16, or even hour 22. So, we keep doing, and trying, and looking. Because you can't always tell, until that final hour...