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Coordinated Universal Time (UTC) Zulu Time
Coordinated Universal Time (U T C) is the basis for civil time today. This 24-hour time standard is kept using highly precise
atomic clocks combined with the Earth’s rotation.

1. Add time zone to local time to obtain UTC (Zulu Time).
2. Subtract time zone number from UTC (Zulu Time) to obtain local time.
The World’s first atomic clock, at the  ammonia absorption  cell  atomic  clock  at  the  National Bureau  of  Standards  (now  the  National Technology) first operated in August 1 9 4 8. Dr. Harold Lyons, inventor is at the right, Dr. Edward U. Condon, Director of the NBS, is at the Left. The ammonia absorption cell is the coil of wave guide surrounding the clock face.
The standard practice is to establish a time zone for each 15° of longitude. This makes a difference of exactly 1 hour between each zone. In the United States, there are four time zones. The time zones are Eastern (75°), Central (90°), Mountain (105°), and Pacific (120°). The dividing lines are somewhat irregular because communities near the boundaries often find it more convenient to use time designations of neighboring communities or trade centers.
Meridians of longitude are drawn from the North Pole to the South
Pole  and are  a t right  angles to the  Equator. The “Prime
Meridian” which passes through Greenwich, England, is used as
the zero line from which measurements  are made in degrees east
(E) and west (W)  to 180°. The  48 conterminous states of  the
United States are between 67° and 125° W longitude.

Time  Zones, the meridians  are  also  useful  for designating time
zones. A day is defined as the time required for the Earth to make
one  complete  rotation  of  360°. Since  the  day is divided into 24
hours, the  Earth revolves at  the  rate of 15° an hour. Noon  is the
time when the sun is directly above a meridian; to the west of that
meridian is morning, to the east is afternoon.