| TIME | SECOND | MINUTE | HOUR | DAY | YEAR | LEAP YEAR | CLOCK | LIGHTYEAR |
| WHAT TIME IS IT...EXACTLY? | DAYLIGHT SAVINGS TIME | UNIVERSAL TIME | CONVERSION CHART | |||||
 atomic clock |
Second. A second is a unit of time equal to one sixtieth of a minute.
Minute. A minute is a unit of time equal to 60 seconds. It is one sixtieth of an hour.
Hour. An hour is an interval of 60 minutes. It is one of 24 equal parts of a day.
Day. A day is the 24-hour period during which Earth completes one rotation on its axis.
Year. A year is the period of time during which Earth completes a single revolution around the Sun. It amounts to 365 days, 5 hours, 49 minutes, and 12 seconds of mean solar time. In the Gregorian calendar we use, the year begins on January 1 and ends on December 31. It is divided into 12 months, 52 weeks, and either 365 or 366 days, depending upon the Leap Year rule. The interval also is known as a calendar year.
Clock. A clock is a freestanding instrument that measures and records time, usually displaying the passage of time through a moving pointer on a dial or else by a digital readout. So-called "atomic clocks" receive signals from Boulder, Colorado, radio station WWVB and synchronize themselves to the NIST-F1 Cesium Fountain Clock at the U.S. National Institute of Standards and Technology Time and Frequency Division. The clock inside a computer is an electronic circuit that generates pulses at a constant rate to synchronize operations.
Leap Year. The rule on leap years according to the U.S. government and the Greenwich Observatory in London: A year is leap if and only if it is divisible by 4, unless it is divisible by 100 in which case it should be divisible by 400, which is the case of the year 2000. Was the year 2000 a leap year? The rule usually holds that years ending in 00 are not leap years. For instance, the years 1700, 1800 and 1900 were not leap years. However, every four hundred years a day has to be added to make our calendar correct. The year 2000 was a leap year ending in 00. The last previous leap year ending in 00 was 1600.
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
- 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
United States Standard Time Law
Universal time. Known as Coordinated Universal Time (UTC) and Greenwich Mean Time (GMT), the universal standard time is the mean solar time at the zero-degree meridian at Greenwich, England. It is used as a basis for finding what time it is throughout most of the world. Here is a universal time FAQ and conversion chart.
What Time Is It...Exactly? The U.S. government's National Institute of Standards and Technology (NIST) is part of the U.S. Department of Commerce. NIST has a Time and Frequency Division located in Boulder, Colorado.
That division maintains the NIST-F1 Cesium Fountain Clock, the nation's standard of time. The cesium clock also is used to create an international time scale.
This NIST-F1 Cesium Fountain Clock measures the natural resonance frequency of cesium atoms and uses that data to define the length of time we call a second.
The cesium clock is so accurate, it neither gains nor loses a second in less than 20 million years.
NIST broadcasts the time from the NIST-F1 Clock across the continental United States through radio station WWV. In Hawaii, the time is broadcast through station WWVH.
So-called "atomic clocks" receive signals from station WWVB and synchronize themselves to the cesium clock.
WWV broadcast schedule and frequencies
U.S. Naval Observatory The U.S. Naval Observatory (USNO) is responsible for precise time determination and dissemination. Its Master Clock is connected to a system of dozens of independently operating cesium atomic clocks and a dozen hydrogen maser clocks distributed across 20 environmentally controlled vaults to ensure stability.
USNO calculates exact time by comparing all of the clocks every 100 seconds. That makes the master clock very reliable and very stable. In fact, its rate does not change by more than about 100 picoseconds (0.0000000001 seconds) per day.
U.S. Naval Observatory Master Clock
Exactly what time is it right now? click here
Lightyear. A lightyear is the distance light travels in one year at the speed of 299,792 kilometers per second. That is 186,282 miles per second. With 31,557,600 seconds in a year, one lightyear equals a distance of 9.46 trillion kilometers or 5.87 trillion miles.
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