TUTORIAL: General Time and Frequency Questions

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What is a leap second?
Why do we need leap seconds?
Why is UTC used as the acronym for Coordinated Universal Time instead of CUT?
When does the next millennium begin?
What is a Modified Julian Date (MJD)?
What is an atomic clock?
Are noon and midnight 12 a.m. or 12 p.m.?
Does the switch to daylight saving time affect UTC?
Is Coordinated Universal Time (UTC) the same thing as Greenwich Mean Time (GMT)?
How is the second defined?
Is the year 2000 a leap year?

Who regulates time zones, in the USA ? What is their history?
Who regulates time zones in Indonesia ?

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What is a leap second?

A leap second is a second added to Coordinated Universal Time (UTC) to make it agree with astronomical time to within 0.9 second. UTC is an atomic time scale, based on the performance of atomic clocks. Astronomical time is based on the rate of rotation of the earth. Since atomic clocks are more stable than the rate at which the earth rotates, leap seconds are needed to keep the two time scales in agreement.

The first leap second was added on June 30, 1972, and they occur at a rate of slightly less than one per year. Although it is possible to have a negative leap second (a second removed from UTC), so far, all leap seconds have been positive (a second has been added to UTC). Based on what we know about the earth's rotation, it is unlikely that we will have a negative leap second in the foreseeable future.

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Why do we need leap seconds?

Leap seconds are introduced to keep the atomic time scale UTC (based on cesium) with the astronomical time scale UT1 (based on the rotation of the earth). Even though the atomic UTC time scale is much more uniform than the UT1 scale, some users must work with earth-based time. For example, boaters navigating by using celestial navigation, need to know the UT1 time of their observations. Since only UTC time is normally broadcast by time-and-frequency radio stations such as WWV, the broadcasters agreed to occasionally adjust UTC time in steps of exactly 1 second so that the difference between UTC and UT1 would never exceed 0.9 seconds. Thus, celestial navigators could then use the received UTC signals and know that any errors with respect to UT1 would be less than 1 second. Leap seconds have been inserted as needed into the UTC time scale since 1972 at average intervals slightly longer than one year.

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Why is UTC used as the acronym for Coordinated Universal Time instead of CUT?

In 1970 the Coordinated Universal Time system was devised by an international advisory group of technical experts within the International Telecommunication Union (ITU). The ITU felt it was best to designate a single abbreviation for use in all languages in order to minimize confusion. Since unanimous agreement could not be achieved on using either the English word order, CUT, or the French word order, TUC, the acronym UTC was chosen as a compromise.

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When does the next millenium begin?

Since a millennium is 1000 years, the first millennium began at the start of the year 1 and ended at the end of the year 1000. The second millennium then began with the year 1001 and will conclude at the end of the year 2000. Therefore, the next millennium begins with the year 2001.

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What is a Modified Julian Date (MJD)?

This is a continuous count of the number of days elapsed since 17 November 1858. It is often more useful than conventional calendar dates for record keeping over long periods of time, since the MJD's of two events can easily be subtracted to determine the time difference in days. Usually, the MJD is specified as a number with 5 significant digits. As an example, the MJD for 1 January 1995 is 49718, meaning that this many days have elapsed between 17 November 1858 and 1 January 1995.

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What is an atomic clock?

An atomic clock is a clock that keeps time using natural characteristic frequencies of atoms, such as cesium, hydrogen, or rubidium. Atomic clocks are extremely stable because the frequencies are almost unaffected by factors like temperature, pressure, or humidity.

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Are noon and midnight 12 a.m. or 12 p.m.?

This is perhaps the trickiest time question of them all. The answer is that the terms 12 a.m. and 12 p.m. are wrong and should not be used.

To illustrate this, consider that "a.m" and "p.m." are abbreviations for "ante meridiem" and "post meridiem." They mean "before noon" and "after noon," respectively. Noon is neither before or after noon; it is simply noon. Therefore, neither the "a.m." nor "p.m." designation is correct. On the other hand, midnight is both 12 hours before noon and 12 hours after noon. Therefore, either 12 a.m. or 12 p.m. could work as a designation for midnight, but both would be ambiguous.

To get around the problem, the terms 12 noon and 12 midnight should be used instead of 12 a.m. and 12 p.m. For example, a bank might be open on Saturday from 8 a.m. to noon. Or a grocery store might be open daily until midnight. If you are making schedules, times such as 12:01 a.m. (1 minute after midnight), or 11:59 p.m. (1 minute before midnight) can also eliminate ambiguity. This method is used by the railroads and airlines.

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Does the switch to daylight saving time affect UTC?

No. UTC, which refers to time on the zero or Greenwich meridian, is not adjusted to reflect either changes either to or from Daylight Saving Time.

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Is Coordinated Universal Time (UTC) the same thing as Greenwich Mean Time (GMT)?

Greenwich Mean Time (GMT) is a 24 hour astronomical time system based on the local time at Greenwich, England. GMT can be considered equivalent to Coordinated Universal Time (UTC) when fractions of a second are not important. However, by international agreement, the term UTC is recommended for all general timekeeping applications, and use of the term GMT is discouraged.

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How is the second defined?

The international definition of a second is "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium atom." This definition was agreed upon in 1967. Atomic clocks based on the cesium atom then became the primary means for accurate timekeeping.

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Is the year 2000 a leap year?

The year 2000 will be a leap year. Century years (like 1900 and 2000) are leap years only if they are evenly divisible by 400. Therefore, 1700, 1800, and 1900 were not leap years, but the year 2000 will be a leap year.

To understand this, you need to know why leap years are necessary in the first place. Leap years are necessary because the actual length of a year is 365.242 days, not 365 days, as commonly stated. Therefore, on years that are evenly divisible by 4 (like 1992, for example) an extra day is added to the calendar on February 29th. However, since the year is slightly less than 365.25 days long, adding an extra day every 4 years results in about 3 extra days being added over a period of 400 years. For this reason, only 1 out of every 4 century years is considered as a leap year

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Who regulates time zones in the USA? What is their history?

In the USA, Time zones are regulated by the United States Department of Transportation (DOT), and not NIST as commonly believed. Time zones were originally controlled by the Interstate Commerce Commission because the need for time zones came about when railroads were first used for interstate commerce. The United States was first divided into four time zones (Eastern, Central, Mountain, and Pacific) on November 18, 1883. In 1967, a congressional act transferred the duties of the Interstate Commerce Commission to the DOT.

In addition, DOT (and not NIST) is also responsible for the rules governing Daylight Saving Time.

Who regulates time zones in Indonesia ?

In Indonesia,  Time zones are regulated by the Indonesian Department of Transportation (Departemen Perhubungan). Puslit KIM-LIPI as NMI, just realizes physical UTC, denoted as UTC(KIM), the physical reference for national time scale determination.

 

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thank's to NIST web site