Universal Time Codes Explained: A Comprehensive Guide

Universal Time (UT) is a time standard fundamentally based on the Earth's rotation. Originally defined as the mean solar time at 0° longitude, UT has evolved to meet the demands of modern science and technology. This article explores the intricacies of UT, its historical development, its relationship with other time standards, and its practical applications in various fields.

The Evolution of Universal Time

Prior to the advent of standardized timekeeping, each municipality determined its local time based on the sun's position. This system sufficed until the rise of railway travel in Britain. The speed of trains necessitated frequent resetting of timepieces over relatively short distances. To address this issue, Britain established Greenwich Mean Time (GMT) in 1847, setting all clocks to the mean solar time at Greenwich, England. This meant that regardless of local solar noon, all clocks in Great Britain adhered to GMT. Telescopes calibrated GMT to the mean solar time at the prime meridian, passing through the Royal Observatory, Greenwich.

As international commerce expanded, the need for a global time standard became evident. Several proposals for a "universal" or "cosmic" time emerged. The development of Universal Time officially began at the International Meridian Conference. On October 22, 1884, the conference recommended the local mean solar time at the Royal Observatory in Greenwich as the base reference for world time, setting the "universal day" to begin at Greenwich mean midnight (0 hours). This decision aligned with the civil Greenwich Mean Time already in use in Great Britain since 1847.

Historically, astronomers began their GMT day at noon, 12 hours after the start of the civil day, to keep all observations made during a single night under the same date. Nautical GMT, used at least until 1805 in the Royal Navy, started 24 hours before astronomical GMT.

Early broadcast time signals were based on UT, reflecting the Earth's rotation. In 1955, the Bureau International de l'Heure (BIH) adopted a proposal by William Markowitz, effective January 1, 1956, dividing UT into UT0, UT1, and UT2. UT0 represented UT as previously computed, UT1 corrected UT0 for polar motion, and UT2 further corrected UT0 for both polar motion and seasonal variations. However, UT0 and UT2 soon faded into obscurity with the introduction of Coordinated Universal Time (UTC).

From 1956, radio station WWV broadcast an atomic clock signal, adjusting it in 20 ms increments to align with UT1. The potential error of up to 20 ms from UT1 was comparable to the differences between UT0, UT1, and UT2. The U.S. Naval Observatory, the Royal Greenwich Observatory, and the UK National Physical Laboratory jointly developed UTC, employing a similar stepping approach.

Today, modern civil time generally follows UTC. However, the term "Greenwich Mean Time" persists in common usage in some countries when referring to UT1, particularly in civil timekeeping, astronomical almanacs, and other references. UTC can serve as a suitable approximation of UT1 when accuracy within one second is sufficient.

Understanding UT1

Universal Time (UT or UT1) is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle with respect to the International Celestial Reference Frame (ICRF), called the Earth Rotation Angle (ERA, which serves as the replacement for Greenwich Mean Sidereal Time). UT1 is the same everywhere on Earth.

The Role of the International Earth Rotation and Reference Systems Service (IERS)

The International Earth Rotation and Reference Systems Service (IERS) plays a crucial role in monitoring the Earth's rotation and UT. The Earth's rotation is somewhat irregular and gradually slowing down due to tidal acceleration. The length of the second was initially determined from observations of the Moon between 1750 and 1890. These factors contribute to the modern mean solar day being slightly longer than the nominal 86,400 SI seconds.

From Ephemeris Time to Terrestrial Time (TT)

As UT's rate is slightly irregular, astronomers introduced Ephemeris Time, which has since been replaced by Terrestrial Time (TT). Barycentric Dynamical Time (TDB), a form of atomic time, is now used in constructing ephemerides of planets and other Solar System objects. These ephemerides are tied to optical and radar observations of planetary motion, and the TDB time scale is fitted so that Newton's laws of motion, with corrections for general relativity, are followed.

UT0, UT1, UT1R, and UT2: A Breakdown

UT0: Universal Time determined at an observatory by observing the diurnal motion of stars or extragalactic radio sources, and also from ranging observations of the Moon and artificial Earth satellites. The location of the observatory is considered to have fixed coordinates in a terrestrial reference frame (such as the International Terrestrial Reference Frame) but the position of the rotational axis of the Earth wanders over the surface of the Earth; this is known as polar motion. UT0 does not contain any correction for polar motion while UT1 does include them.
UT1: Includes corrections for polar motion, making it a more accurate representation of Earth's rotation. The difference between UT0 and UT1 is on the order of a few tens of milliseconds.
UT1R: A smoothed version of UT1, filtering out periodic variations due to tides.
UT2: A further smoothed version of UT1, filtering out periodic seasonal variations. It is mostly of historic interest and rarely used anymore.

Coordinated Universal Time (UTC): The Modern Standard

Coordinated Universal Time (UTC) serves as the primary global time standard, regulating clocks and time worldwide. It provides a reference for the current time and forms the basis for civil time and time zones. UTC is based on International Atomic Time (TAI), a weighted average of hundreds of atomic clocks around the world. The coordination of time and frequency transmissions globally began on January 1, 1960. Since its adoption, UTC has been adjusted several times, notably with the addition of leap seconds starting in 1972.

The Debate Over Leap Seconds

Recent years have seen significant discussions about eliminating leap seconds from the timekeeping system. Leap seconds occasionally disrupt timekeeping systems worldwide. The official abbreviation for Coordinated Universal Time is UTC. In 1967 the CCIR adopted the names Coordinated Universal Time and Temps Universel Coordonné for the English and French names with the acronym UTC to be used in both languages. The name "Coordinated Universal Time (UTC)" was approved by a resolution of IAU Commissions 4 and 31 at the 13th General Assembly in 1967 (Trans.

UTC in Practice

The westernmost time zone uses UTC−12, being twelve hours behind UTC; the easternmost time zone uses UTC+14, being fourteen hours ahead of UTC. UTC is used in many Internet and World Wide Web standards. The Network Time Protocol (NTP), designed to synchronize the clocks of computers over the Internet, transmits time information from the UTC system. If only milliseconds precision is needed, clients can obtain the current UTC from a number of official internet UTC servers. UTC is also the time standard used in aviation, e.g., for flight plans and air traffic control. In this context, it is frequently referred to as Zulu time. Weather forecasts and maps all use UTC to avoid confusion about time zones and daylight saving time.

UTC divides time into days, hours, minutes, and seconds. Days are conventionally identified using the Gregorian calendar, but Julian day numbers can also be used. Each day contains 24 hours and each hour contains 60 minutes. The number of seconds in a minute is usually 60, but with an occasional leap second, it may be 61 or 59 instead. Thus, in the UTC time scale, the second and all smaller time units (millisecond, microsecond, etc.) are of constant duration, but the minute and all larger time units (hour, day, week, etc.) are of variable duration. Nearly all UTC days contain exactly 86,400 SI seconds with exactly 60 seconds in each minute.

UTC is within about one second of mean solar time (such as UT1) at 0° longitude, (at the IERS Reference Meridian). The mean solar day is slightly longer than 86,400 SI seconds so occasionally the last minute of a UTC day is adjusted to have 61 seconds. The extra second is called a leap second. It accounts for the grand total of the extra length (about 2 milliseconds each) of all the mean solar days since the previous leap second. The last minute of a UTC day is permitted to contain 59 seconds to cover the remote possibility of the Earth rotating faster, but that has not yet been necessary.

Since 1972, UTC may be calculated by subtracting the accumulated leap seconds from International Atomic Time (TAI), which is a coordinate time scale tracking notional proper time on the rotating surface of the Earth (the geoid). In order to maintain a close approximation to UT1, UTC occasionally has discontinuities where it changes from one linear function of TAI to another. These discontinuities take the form of leap seconds implemented by a UTC day of irregular length. Discontinuities in UTC occurred only at the end of June or December.

As with TAI, UTC is only known with the highest precision in retrospect. Users who require an approximation in real time must obtain it from a time laboratory, which disseminates an approximation using techniques such as GPS or radio time signals. Because of time dilation, a standard clock not on the geoid, or in rapid motion, will not maintain synchronicity with UTC.

It is impossible to compute the exact time interval elapsed between two UTC timestamps without consulting a table showing how many leap seconds occurred during that interval. By extension, it is not possible to compute the precise duration of a time interval that ends in the future and may encompass an unknown number of leap seconds (for example, the number of TAI seconds between "now" and 2099-12-31 23:59:59). Therefore, many scientific applications that require precise measurement of long (multi-year) intervals use TAI instead. TAI is also commonly used by systems that cannot handle leap seconds.

UTC Offsets and Time Zones

Time zones are usually defined as differing from UTC by an integer number of hours, although the laws of each jurisdiction would have to be consulted if sub-second accuracy was required. The time zone using UTC is sometimes denoted UTC+00:00 or by the letter Z-a reference to the equivalent nautical time zone (GMT), which has been denoted by a Z since about 1950. Time zones were identified by successive letters of the alphabet and the Greenwich time zone was marked by a Z as it was the point of origin. The letter also refers to the "zone description" of zero hours, which has been used since 1920 (see time zone history). Since the NATO phonetic alphabet word for Z is "Zulu", UTC is sometimes known as "Zulu time".

UTC does not change with a change of seasons, but local time or civil time may change if a time zone jurisdiction observes daylight saving time (summer time).

The Atomic Age of Timekeeping

In 1955, the caesium atomic clock was invented. This provided a form of timekeeping that was both more stable and more convenient than astronomical observations. Naval Observatory started to develop atomic frequency time scales; by 1959, these time scales were used in generating the WWV time signals, named for the shortwave radio station that broadcasts them. In a controversial decision, the frequency of the signals was initially set to match the rate of UT, but then kept at the same frequency by the use of atomic clocks and deliberately allowed to drift away from UT. When the divergence grew significantly, the signal was phase shifted (stepped) by 20 ms to bring it back into agreement with UT.

In 1958, data was published linking the frequency for the caesium transition, newly established, with the ephemeris second. The ephemeris second is a unit in the system of time that, when used as the independent variable in the laws of motion that govern the movement of the planets and moons in the Solar System, enables the laws of motion to accurately predict the observed positions of Solar System bodies. Within the limits of observable accuracy, ephemeris seconds are of constant length, as are atomic seconds.

The coordination of time and frequency transmissions around the world began on January 1, 1960. In 1961, the Bureau International de l'Heure began coordinating the UTC process internationally (but the name Coordinated Universal Time was not formally adopted by the International Astronomical Union until 1967). From then on, there were time steps every few months, and frequency changes at the end of each year. The jumps increased in size to 0.1 seconds.

Defining the Second

In 1967, the SI second was redefined in terms of the frequency supplied by a caesium atomic clock. The length of second so defined was practically equal to the second of ephemeris time. This was the frequency that had been provisionally used in TAI since 1958. It was soon decided that having two types of second with different lengths, namely the UTC second and the SI second used in TAI, was a bad idea. It was thought better for time signals to maintain a consistent frequency, and that this frequency should match the SI second. Thus it would be necessary to rely on time steps alone to maintain the approximation of UT.

There was also dissatisfaction with the frequent jumps in UTC (and SAT). In 1968, Louis Essen, the inventor of the caesium atomic clock, and G. M. R. Winkler both independently proposed that steps should be of 1 second only. to simplify future adjustments. This system was eventually approved as leap seconds in a new UTC in 1970 and implemented in 1972, along with the idea of maintaining the UTC second equal to the TAI second.

As an intermediate step at the end of 1971, there was a final irregular jump of exactly 0.107758 TAI seconds, making the total of all the small time steps and frequency shifts in UTC or TAI during 1958-1971 exactly ten seconds, so that 1 January 1972 00:00:00 UTC was 1 January 1972 00:00:10 TAI exactly, and a whole number of seconds thereafter. At the same time, the tick rate of UTC was changed to exactly match TAI. UTC also started to track UT1 rather than UT2. The first leap second occurred on 30 June 1972. Since then, leap seconds have occurred on average about once every 19 months, always on 30 June or 31 December.

The Slowing of the Earth and the Need for Leap Seconds

Earth's rotational speed is very slowly decreasing because of tidal deceleration; this increases the length of the mean solar day. The length of the SI second was calibrated on the basis of the second of ephemeris time and can now be seen to have a relationship with the mean solar day observed between 1750 and 1892, analyzed by Simon Newcomb. As a result, the SI second is close to ⁠1/86400⁠ of a mean solar day in the mid‑19th century. In earlier centuries, the mean solar day was shorter than 86,400 SI seconds, and in more recent centuries it is longer than 86,400 seconds.

The excess of the LOD over the nominal 86,400 s accumulates over time, causing the UTC day, initially synchronized with the mean sun, to become desynchronized and run ahead of it. It will take about 50,000 years for a mean solar day to lengthen by one second (at a rate of 2 ms per century). This rate fluctuates within the range of 1.7-2.3 ms/cy.

Leap seconds are timed to keep DUT1 within the vertical range depicted by the adjacent graph. The frequency of leap seconds therefore corresponds to the slope of the diagonal graph segments, and thus to the excess LOD. As the Earth's rotation continues to slow, positive leap seconds will be required more frequently. The long-term rate of change of LOD is approximately +1.7 ms per century. At the end of the 21st century, LOD will be roughly 86,400.004 s, requiring leap seconds every 250 days. Over several centuries, the frequency of leap seconds will become problematic.

Potential for Negative Leap Seconds

A change in the trend of the UT1 - UTC values was seen beginning around June 2019 in which instead of slowing down (with leap seconds to keep the difference between UT1 and UTC less than 0.9 seconds) the Earth's rotation has sped up, causing this difference to increase. If the trend continues, a negative leap second may be required, which has not been used before. Some time in the 22nd century, two leap seconds will be required every year. The current practice of only allowing leap seconds in June and December will be insufficient to maintain a difference of less than 1 second, and it might be decided to introduce leap seconds in March and September.

The Future of UTC: Abolishing Leap Seconds

In 2022 a resolution was adopted by the General Conference on Weights and Measures to redefine UTC and abolish leap seconds, but keep the civil second constant and equal to the SI second, so that sundials would slowly get further and further out of sync with civil time. The leap seconds will be eliminated by 2035. The resolution does not break the connection between UTC and UT1, but increases the maximum allowable difference. The details of what the maximum difference will be and how corrections will be implemented is left for future discussions.

This will result in a shift of the sun's movements relative to civil time, with the difference increasing quadratically with time (i.e., proportional to elapsed centuries squared). This is analogous to the shift of seasons relative to the yearly calendar that results from the calendar year not precisely matching the tropical year length. This would be a change in civil timekeeping, and would have a slow effect at first, but becoming drastic over several centuries. UTC (and TAI) would be more and more ahead of UT; it would coincide with local mean time along a meridian drifting eastward faster and faster. Thus, the time system will lose its fixed connection to the geographic coordinates based on the IERS meridian.

Time Zone Offsets and Calculations

Every time zone is defined by its difference from UTC, known as its UTC offset. For example, the local time in China is eight hours ahead of UTC, so China Standard Time has a UTC offset of UTC+8. When it’s 09:00 UTC, clocks in Beijing show 17:00 (5:00 pm).

Shifting UTC Offsets

While every time zone has a fixed UTC offset, some countries jump back and forth from one time zone to another every year, so the local UTC offset shifts accordingly. This occurs in countries that observe Daylight Saving Time (DST), where the UTC offset changes when clocks spring ahead or fall back. For example, New York follows Eastern Standard Time (EST) during part of the year, which is five hours behind UTC (UTC-5). In early March, clocks go ahead by one hour to Eastern Daylight Time (EDT), reducing the offset to UTC-4.

24-Hour Notation

UTC always uses the 24-hour clock, also known as military time. For example, 7 o’clock in the evening must be written as 19:00 UTC. It is incorrect to write 7:00 pm UTC.

Measuring UTC

Two components are used to keep UTC ticking at exactly the right pace:

International Atomic Time (TAI): A time scale that combines the output of some 450 highly precise atomic clocks worldwide.
Universal Time (UT1): Also known as astronomical time or solar time, it refers to the Earth’s rotation. It is used to compare the pace provided by TAI with the actual length of a day on Earth.

To reconcile the ultra-precise speed of atomic clocks with the fluctuations of Earth’s rotation, a leap second is added to or removed from UTC when the difference between the two components is predicted to reach 0.9 seconds within 12 months.

Greenwich Mean Time (GMT)

Although UTC in its present form has only been around for a few decades, the general idea of using a universal time standard to anchor local times worldwide was first put into practice as early as 1884: At the International Meridian Conference, delegates from 25 countries designated Greenwich Mean Time (GMT) as the reference point. This historical decision is why GMT is still occasionally used as a substitute for UTC today. At the time, GMT was based on Earth’s rotation-specifically, on the mean solar time at the prime meridian, which runs through the Royal Observatory in Greenwich, London. With advances in timekeeping technology and the need for a more precise and stable global time standard, UTC eventually replaced GMT.

A Universal Time (AUT) - Roblox Game

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