An hour is made of 60 minutes

Not if you cross a leap second (made even more fun by their non-representation in POSIX time).

(ed: Well, of course, it depends on what you actually mean by 'minutes', whether an hour with a positive leap second has 60 minutes or 60.166... minutes. But that just leads into the need to figure out exactly what question you're actually asking at any given moment. Stupid inconsistent world :)