Oh I have a great one: > days don't overlap If you ever have the pleasure of working with the Jewish religious calendar, it defines day boundaries by sundown

Oh I have a great one: > days don't overlap If you ever have the pleasure of working with the Jewish religious calendar, it defines day boundaries by sundown and rise of the first star, which are not simultaneous. This leads to a phase during which two days co-exist. And of course it's not continuous over the year and by location.