Well… it's an arbitrary standard. We just call a day one full rotation of the earth, and we just happen to divide it into 24hr periods with 60 subdivisions. Th

Well… it's an arbitrary standard. We just call a day one full rotation of the earth, and we just happen to divide it into 24hr periods with 60 subdivisions. Theoretically, you could measure the rotation of the earth and call that a day, but I have no idea how you would measure that down to the millisecond. As to "Real Time", I don't know about the specific legalities, but the standard goes as follows: everyone uses UTC http://en.wikipedia.org/wiki/Coordinated_Universal_Time http://en.wikipedia.org/wiki/Coordinated_Universal_Time which is based off the International Atomic Time http://en.wikipedia.org/wiki/International_Atomic_Time http://en.wikipedia.org/wiki/International_Atomic_Time I too would be interested in how they picked the beginning epoch, i.e. "this moment right here is midnight from which all other seconds shall be referenced against" but in the grand scheme of things it doesn't matter - the only thing that matters is the standard we agreed to measure against.