This means you advance the clock 24 hours in essence you advance one day on the calendar.

Moving west across the International Date Line one moves from the -12 UTC time zone to the +12 UTC time zone. At the meeting of the -12 and +12 time zones is the International Date Line. Time zone offsets are identified as -12 UTC through 0 to +12 UTC with the minus values signifying time 'before' or ahead of prime meridian (which is the Western Hemisphere). However, today the individual zone boundaries are no longer straight, nor are they always continuous, as they have been modified for convenience and to satisfy the desires of sovereign nations. With the 360° daily rotation of the earth, the sun is moving 15° each hour which leads to the formation of 24 time zones. Today, one can visit the Royal Observatory and straddle this 0° meridian with one foot in each hemisphere. This 0° meridian divides the Eastern from the Western Hemisphere.

This was for both for longitude and timekeeping. settled the matter where it was agreed to establish a single 'prime' meridian (0° longitude), passing through Greenwich. In 1884, a conference in Washington, D.C.

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