Since radio signals can cross multiple time zones and the international date line, some worldwide standard for time and date is needed. This standard is Coordinated Universal Time, abbreviated UTC. This was formerly known as Greenwich mean time (GMT). Other terms used to refer to it include “Zulu time” (after the “Z” often used after UTC times), “universal time,” and “world time.”UTC is used by international short-wave broadcasters in their broadcast and program schedules. Ham radio operators, short-wave listeners, the military, and utility radio services are also big users of UTC.
Greenwich mean time was based upon the time at the zero degree meridian that crossed through Greenwich, England. GMT became a world time and date standard because it was used by Britain’s Royal Navy and merchant fleet during the nineteenth century. Today, UTC uses precise atomic clocks, short-wave time signals, and satellites to ensure that UTC remains a reliable, accurate standard for scientific and navigational purposes. Despite the improvements in accuracy, however, the same principles used in GMT have been carried over into UTC.
UTC uses a 24-hour system of time notation. “1:00 a.m.” in UTC is expressed as 0100, pronounced “zero one hundred.” Fifteen minutes after 0100 is expressed as 0115; thirty-eight minutes after 0100 is 0138 (usually pronounced “zero one thirty-eight”). The time one minute after 0159 is 0200. The time one minute after 1259 is 1300 (pronounced “thirteen hundred”). This continues until 2359. One minute later is 0000 (“zero hundred”), and the start of a new UTC day.
To convert UTC to local time, you have to add or subtract hours from it. For persons west of the zero meridian to the international date line, hours are subtracted from UTC to convert to local time.