Figure 2-2 shows a standard time zone chart of the world. Each sector appears as a
vertical band 15° of longitude in width. Notice that each zone on the chart is defined by
the number of hours of difference between the time kept within that zone and the time
kept within the zone centered on the prime (0°) meridian, passing through Greenwich,
England. Each zone is labeled with letters, called time zone indicators that assist in
identification of the zones.
Time based upon the relationship of the mean sun to the prime meridian is called
Coordinated Universal Time (UTC). It is also referred to as ZULU time because of its
time zone indicator letter (Z). UTC is accurate to approximately a nanosecond (billionth
of a second) per day.
The farther west of Greenwich that a time zone lies, the earlier that zone will be in
relation to UTC. A plus (+) sign in front of the hourly difference figure indicates that the
hours must be added to the local zone time to convert it to UTC. The farther east of
Greenwich a time zone is located, the later its time will be relative to UTC. A minus (-)
sign indicates that the hours must be subtracted to obtain UTC. The Greenwich zone
extends 7 1/2° either side of the prime meridian. A new time zone boundary lies every
15° thereafter, across both the Eastern Hemisphere and the Western Hemisphere,
resulting in the twenty-fourth zone being split into two halves by the 180th meridian. The
half on the west side of this meridian keeps time 12 hours behind UTC, making its
difference + 12, while the half on the east side is -12. These zones are numbered + 1
through + 12 to the west of the Greenwich zone and -1 through -12 to the east.