Universal Time FAQ
Does UTC change with the seasons? No, it doesn't change, but we change our habits. So, we have to adjust our local clocks and watches accordingly.

• In many places in the spring of the year, we set our clocks ahead one hour. Americans say, "spring ahead" or "spring forward."
• In the fall of the year, we set our timepieces back. Americans say, "fall back."

How can I avoid the international confusion over time? Use coordinated universal time (UTC). To avoid further confusion between morning and afternoon times -- a.m. and p.m. -- use the 24-hour military-style clock. For instance, 1:00 p.m. becomes 1300 in military time and 11:00 p.m. becomes 2300 in military time. After noon, add 12 to the hour to convert to military time.

How do I convert dates? In converting times, you must pay attention to the correct date as the time crosses midnight or the International Date Line. A world map can help you picture the International Date Line time and see when a date conversion is needed. Daylight Savings Time (DST) also can confuse dates, since the local standard time is advanced one hour for DST.

What is Greenwich Mean Time? For thousands of years, people have measured time based on the position of the Sun. It is noon when the Sun is highest in the sky. Sundials were the main source of time information into the Middle Ages. Then, mechanical clocks began to appear. Cities had town clocks, which would be set by measuring the position of the Sun. Every city was on a slightly different time.

Great Britain became the first country to set a standard time throughout a region when it established the Greenwich Mean Time standard in the 1840s. The railways cared most about the inconsistencies of local time. They forced the uniform time on the country. A railway adopted the time in 1840 and by 1847 most used it. By 1855, most public clocks in Britain were set to GMT, although some had two minute hands, one for local time and one for GMT. The legal system adopted GMT in 1880.

Because Great Britain was the major world power at the time, it placed the center of the first time zone at England's Royal Greenwich Observatory, which was located on the 0-degree longitude meridian. That line was determined by the Astronomer Royal using a transit telescope. The international date line was set at the 180-degree longitude meridian in the Pacific. As time pieces became more accurate and communication became global, there needed to be a point from which all other world times were based. Since Great Britain was the world's foremost martime power when the concept of latitude and longitude came to be, the starting point for designating longitude was the "prime meridian" which is zero degrees and runs through the Royal Greenwich Observatory, in Greenwich, England, just outside London. As a result, when the concept of time zones was introduced, the "starting" point for calculating the different time zones was/is at the Royal Greenwich Observatory. When it is noon at the observatory, it is five hours earlier (under Standard Time) in Washington, D.C.; six hours earlier in Chicago; seven hours earlier in Denver; and, eight hours earlier in Los Angeles.

When did GMT become international? The International Meridian Conference met in Washington, D.C. on November 1, 1884, with delegates from 25 countries. It established time zones with a one hour difference between adjacent time zones. The zones referenced mean solar time to the 24 standard meridians, based 15 degrees east and west of Greenwich, the point from which reckoning for each day should begin.

What about time zones in the United States? Time zones in the U.S. weren't considered necessary until trains criss-crossed the country. Before that, cities relied on their own local "Sun time." Of course, there were a lot of problems associated with tracking hundreds of local times and publishing timetables. These problems were overcome partially by the establishment of some 100 different, but consistent, railroad time zones.

The United States government fixed the problem by dividing the country into four time zones. At noon on November 18, 1883, the master clock at the United States Naval Observatory (USNO) transmitted the time by telegraph lines to major cities, each of which adjusted their clocks to their time zone's correct time. The railroads then began using the standard time zones for their schedules.

Congress adopted standard time zones into law in 1918 and made the Interstate Commerce Commission responsible for changes. When Congress created the Department of Transportation in 1966, it transferred the responsibility for time there.   world time zones map

When did UTC start? Several groups of time scales existed until 1972. Each was offset from the others and constructed for special purposes, such as navigation and satellite tracking. The proliferation of time scales posed a problem because Earth's rotation affected UTC. It didn't affect atomic time, so UTC was developed as a compromise time scale effective January 1, 1972.

What is military time? Military personnel avoid confusion between morning and afternoon times -- a.m. and p.m. -- by using a 24-hour clock, rather than a 12-hour clock. After noon, add 12 to the hour to convert to military time. For example, 1:00 p.m. becomes 1300 in military time and 11:00 p.m. becomes 2300 in military time. This is also known as government time.

What is government time? Government workers also avoid confusion between a.m. and p.m. by using a 24-hour clock. It's the same as military time.

What is zulu time? The name "zulu time" comes from the way military personnel write UTC. The written 24-hour time is followed by the letter "z" to indicate that it is UTC. The letter "z" was chosen because of the "z" in the old Greenwich Mean Time (GMT) "zero meridian." The letter "z" in the phonetic alphabet is spoken as "zulu." Military sometimes also shorten the expression to "z-time."

By the way, if the time written is local standard time, and not universal time, military personnel, civilian aviators and other communicators use these letters to refer to the U.S. time zones:

U.S. time zone	letter	phonetic
Eastern	R	romeo
Central	S	sierra
Mountain	T	tango
Pacific	U	uniform
Alaska	V	victor
Hawaii	W	william
UTC	Z	zulu

Who uses universal time? Major users of highly precise universal time include astronomers, spacecraft tracking stations, science labs, military and civilian ships and aircraft, radio and TV stations, seismographers, geologists, power companies and ham radio operators.

What is daylight savings time? Efforts to save daylight have been made in the United States and Europe since World War I, when the daylight savings time system first was adopted to conserve fuel for electric power for the war effort. During later times when there were no federal laws creating daylight saving time in the U.S., states and localities were free to observe it or not. This caused confusion for the broadcasting industry as well as for railways, airlines and bus companies. There was a hodgepodge of time observances and no agreement when to change clocks.

The Uniform Time Act signed in 1966 established the national daylight saving time, although any state that wanted to be exempt could do so by passing a state law. The Uniform Time Act established a uniform system for daylight saving time within each time zone throughout the U.S.

The Federal law was amended in 1986. Today, daylight saving time in the United States:

• begins at 2 a.m. local time on the first Sunday of April
• ends at 2 a.m. local time on the last Sunday of October

In most countries of western Europe, daylight saving time:

• begins at 1 a.m. UTC on the last Sunday of March and
• ends at 1 a.m. UTC on the last Sunday of October

Observance of daylight saving time varies elsewhere in the world.

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