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Neither!

Twelve midnight A.M. and twelve midnight P.M., or 00:00 A.M. and 00:00 P.M., mean nothing at all. They are simply the midpoints that divide the day into two equal halves.

Each and every day begins exactly at midnight, and each A.M. begins precisely thereafter. Similarly, each P.M. begins immediately after noon. No meaning can be assigned to 12:00 A.M. (00:00 A.M.), or to 12:00 P.M. (00:00 P.M.). They are merely reference points meant to simplify timetables for us.

Along the same line, the Universal Day, established by the International Convention in 1884 in Washington, D.C., U.S.A., operates according to World Time, or Universal Time at Greenwich, England. The logic regarding Midnight and Noon also applies to Greenwich Mean Time, commonly referred to as GMT or Zulu time; Midnight and Noon represent markers, or "page breaks" in the day and in the night, and may be represented by 00:00 o抍lock.

Greenwich, England also holds the distinction of being at the point of zero longitude, where East meets West. The 1884 international agreement also recognized this line of zero degrees longitude as the prime meridian, a point from which all points on the earth抯 surface are measured.