The Prime Meridian (0°) passes through the Royal Astronomical Observatory at Greenwich village, near London. So, in 1884, it was decided by all countries to adopt the local time of Prime Meridian (0°) worldwide.
Hence, when sun is directly overhead at 0° longitude and it is 12 o'clock (noon) there, the international time is calculated east or west of the Prime Meridian with reference to this time.