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, shortwave 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 (which includes all of North America), hours are subtracted from UTC to convert to local time. Below is a table showing the number of hours to subtract from local time zones in North America in order to convert UTC to local time:

The second table is direct conversion from UTC to U.S.A. timezones.

A major source of confusion when using UTC is that the date also follows UTC. Suppose your local time zone is Central standard, and you want to hear a shortwave program scheduled to be broadcast at 0400 UTC Saturday. You do the math, and find that 0400 UTC is equal to 10:00 p.m. Central standard time. If you tune in at 10:00 p.m. on Saturday, however, you won't hear the program. Since the date is also UTC, you need to listen at 10:00 p.m. Friday to hear the program.