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Zone time A time system used in commercial and government navigation which changes time zones at specific longitudes, regardless of what is going on at that place. The central time meridian of each zone is a whole multiple of 15°, called the zone description of that zone. The boundaries of each zone are 7° 30' of longitude to either side of the central meridian. According to the Zone Time system, every point on earth between 127° 30' W and 112° 30' W keeps time that is 8 hours behind Greenwich mean time (ZD = +8h, since 8 x 15 = 120°). There is no daylight saving time in this system, and it is assumed that all ship's clocks change by 1 hour when a time zone boundary is crossed. Zone time is like standard time with straight boundaries along specific meridians and no daylight changes. See also standard time, watch time, and time zone. See: sextant sight, standard time, time zone, universal time, watch error, watch rate, watch time, zone description, and zone time. See also: Time Keeping in Navigation and Weather. Abbreviation:  ZT

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