United world time. UTC: what does it mean, how is it decrypted? Difference between UTC and GMT time zones

The basis modern system time zones are required Coordinated Universal Time (UTC)(universal time), which determines the time of all time zones.

Local time in many countries of the Northern Hemisphere (but not in all) increases by 1 hour in summer (in the Southern Hemisphere it decreases by 1 hour at the same time), and returns to normal standard time in winter, which often also changes. Due to these seasonal and non-seasonal changes in local time in radio broadcasting, international transport, radio communications, e-mail and other international means of communication, there is a huge confusion about the docking of time between different countries.

UTC time is not translated in winter and summer, so for those places where there is a translation into summer time, the UTC offset changes.

Initial (zero) meridian - Greenwich meridian with geographic longitude equal to 0 ° 00 "00", divides the globe into the western and eastern hemispheres. Passes through the former Greenwich Observatory (in the suburbs of London)

GMT(Greenwich Mean Time) - "Greenwich Mean Time" - on the Greenwich meridian. It is determined by astronomical observations of the daily movement of stars. It is unstable (within a second per year) and depends on the constant change in the speed of the Earth's rotation, the movement of the geographic poles on its surface and the nutation of the planet's axis of rotation. Greenwich (astronomical) time - GMT is close in value to UTC (atomic time), and for the time being will be used as its synonym. Another name - "ZULU time"

In Russian-speaking meteorology, GMT is denoted as GMT (Greenwich Mean / or Geographic / Time)

Unlike GMT, UTC is set using atomic clocks. The UTC time scale has been introduced since 1964 to harmonize UT1 (astronomical measurements) and TAI (atomic clocks).

Since 1900, the mean solar day has increased by 0.002 atomic seconds, and therefore Greenwich Mean Time diverges from international atomic time by about 1 second every 500 days. Taking into account this progressive phase shift between the two time scales, and without abandoning the high accuracy offered by atomic clocks, a compromise was reached in 1972 that led to the definition of the concept of Coordinated Universal Time (UTC), which is now used as the official measure of time in the world. . In essence, UTC time runs like international atomic time, and when the difference with Greenwich time reaches 1 second, 1 second is added to the UTC time scale, which is called jumping (leap second). Thus, the difference is always kept below 0.9 sec. The addition of the jump second is reported by the International Earth Rotation Service (IERS), which continuously monitors the rate of rotation. The best dates to add a jump second are June 30th and December 31st. By the way, the term UTC is also a compromise between the English CUT (Coordinated Universal Time) and the French TUC (Temps Universel Coordlnaire).

Coordinated Universal Time (UTC) always remains an independent reference for the whole world and from which, knowing the difference between your standard time, you can always calculate your local time.

Precise time signals are transmitted by radio, television, the Internet - in the UTC system.

Time difference with the countries of the world, regions of Russia and Moscow.

Time is now set using Coordinated Universal Time (UTC), which was introduced to replace Greenwich Mean Time (GMT). The UTC scale is based on the Uniform Atomic Time Scale (TAI) and is more convenient for civilian use. around the globe are expressed as positive and negative offsets from UTC. It should be remembered that UTC time is not translated either in winter or in summer. Therefore, for those places where there is a daylight saving time, the offset relative to UTC changes.

Principles of demarcation
The modern system is based on universal coordinated time (universal time), on which the time of all depends. In order not to enter local time for each degree (or each minute) of longitude, the surface of the Earth is conventionally divided by 24. When switching from one to another, the values ​​of minutes and seconds (time) are preserved, only the value of hours changes. There are some countries in which local time differs from world time not only by a whole number of hours, but by an additional 30 or 45 minutes. True, such time zones are not standard.

Russia - 11 time zones;
Canada - 6 time zones;
USA - 6 time zones (including Hawaii, excluding island territories: American Samoa, Midway, Virgin Islands, etc.);
in the autonomous territory of Denmark - Greenland - 4 time zones;
Australia and Mexico - 3 time zones each;
Brazil, Kazakhstan, Mongolia and the Democratic Republic of the Congo - 2 time zones each.
The territories of each of the remaining countries of the world are located in only one of any time zone.

Despite the fact that the territory of China is located in five theoretical periods, a single Chinese standard time applies throughout its territory.

The only administrative-territorial unit in the world, the territory of which is divided into more than two, is the Republic of Sakha (Yakutia), which is a subject of the Russian Federation (3 time zones).

In the USA and Canada, the borders are very tortuous: there are often cases when they go through a state, province or territory, since territorial belonging to a particular belt is determined at the levels of administrative-territorial units of the second order.

UTC-12 - International Date Line
UTC-11 - Samoa
UTC-10 - Hawaii
UTC-9 - Alaska
UTC-8 - North American Pacific Time (US and Canada)
UTC-7 - Mountain time (USA and Canada), Mexico (Chihuahua, La Paz, Mazatlán)
UTC-6 - Central Time (USA and Canada), Central American Time, Mexico (Guadalajara, Mexico City, Monterrey)
UTC-5 - Eastern North American Time (USA and Canada), South American Pacific Time (Bogotá, Lima, Quito)
UTC-4:30 - Caracas
UTC-4 - Atlantic Time (Canada), South American Pacific Time, La Paz, Santiago)
UTC-3:30 - Newfoundland
UTC-3 - South American Eastern Time (Brazilia, Buenos Aires, Georgetown), Greenland
UTC-2 - Mid-Atlantic Time
UTC-1 - Azores, Cape Verde
UTC+0 - Western European Time (Dublin, Edinburgh, Lisbon, London, Casablanca, Monrovia)
UTC+1 - Central European Time (Amsterdam, Berlin, Bern, Brussels, Vienna, Copenhagen, Madrid, Paris, Rome, Stockholm, Belgrade, Bratislava, Budapest, Warsaw, Ljubljana, Prague, Sarajevo, Skopje, Zagreb) West Central African Time
UTC+2 - Eastern European Time (Athens, Bucharest, Vilnius, Kiev, Chisinau, Minsk, Riga, Sofia, Tallinn, Helsinki, Kaliningrad), Egypt, Israel, Lebanon, Turkey, South Africa
UTC+3 - Moscow time, East African time (Nairobi, Addis Ababa), Iraq, Kuwait, Saudi Arabia
UTC+3:30 - Tehran time
UTC+4 - Samara Time, United Arab Emirates, Oman, Azerbaijan, Armenia, Georgia
UTC+4:30 - Afghanistan
UTC+5 - Yekaterinburg Time, West Asian Time (Islamabad, Karachi, Tashkent)
UTC+5:30 - India, Sri Lanka
UTC+5:45 - Nepal
UTC+6 - Novosibirsk, Omsk Time, Central Asian Time (Bangladesh, Kazakhstan)
UTC+6:30 - Myanmar
UTC+7 - Krasnoyarsk time, Southeast Asia(Bangkok, Jakarta, Hanoi)
UTC+8 - Irkutsk Time, Ulaanbaatar, Kuala Lumpur, Hong Kong, China, Singapore, Taiwan, Western Australian Time (Perth)
UTC+9 - Yakut time, Korea, Japan
UTC+9:30 Central Australian Time (Adelaide, Darwin)
UTC+10 - Vladivostok Time, East Australian Time (Brisbane, Canberra, Melbourne, Sydney), Tasmania, Western Pacific Time (Guam, Port Moresby)
UTC+11 - Magadan Time, Central Pacific Time (Solomon Islands, New Caledonia)
UTC+12 - Kamchatka time, Marshall Islands, Fiji, New Zealand
UTC+13 - Tonga
UTC+14 - Line Islands (Kiribati)

Prior to the introduction of standard time, each city used its own local solar time, depending on geographic longitude. The standard time system was adopted at the end of the 19th century as an attempt to end the confusion caused by using its own solar time in any given locality. The need to introduce such a standard became extremely urgent with the development of the railway, if train schedules were drawn up according to the local time of each city, which caused not only inconvenience and confusion, but also frequent accidents. This was especially true for large areas connected by the railway system.

Before the invention of the railroad, traveling from one place to another took so much time. When traveling, time would only need to be adjusted by 1 minute every 12 miles. But with the advent of the railroad, which made it possible to cover hundreds of miles a day, timing became a serious problem.

Great Britain

Britain was the first country to decide on the establishment of one standard time throughout the country. The problem of inconsistency in local time was dealt with more by British Railways, which forced the government to unify time throughout the country. The idea was originally owned by Dr. William Hyde Wollaston (1766-1828) and was taken up by Abraham Follett Osler (1808-1903). The time was set to Greenwich Mean Time (GMT) and for a long time it was called "London time".

The first to use "London time" (1840) was the Great Western Railway. Others began to imitate it, and by 1847 most British railways were already using singular time. On September 22, 1847, the Railway Clearing House, which set standards for the entire industry, recommended that GMT be set at all stations with the permission of the General Postal Service. The transition took place on December 1, 1847.

On August 23, 1852, time signals were first transmitted by telegraph from the Royal Greenwich Observatory.

Before 1855, the vast majority of public clocks in Britain were set to Greenwich Mean Time. But the process of official transition to new system The timing was held back by British legislation, which kept local time officially accepted for many more years. This led, for example, to such oddities as, for example, polling stations opened at 08:13 and closed at 16:13. Officially, the transition to a new time in Britain nevertheless took place after the introduction of a legislative act on determining the time on August 2, 1880.

New Zealand

New Zealand was the first country to officially adopt standard time throughout the country (November 2, 1868). The country is located 172° 30" longitude east of Greenwich and its time was 11 hours 30 minutes ahead of Greenwich Mean Time. This standard was known as New Zealand Mean Time.

North America

In America and Canada standard time was also introduced on November 18, 1883 railways. By then, timing was a local matter. Most cities used "solar time" and the standard by which the time was set was often some well-known clock in each locality (for example, clocks on church bell towers or in jewelry store windows.

The first person in the United States to feel the growing need for time standardization was the amateur astronomer William Lambert, who in early 1809 submitted a recommendation to Congress for the establishment of time meridians in the country. But this recommendation was rejected, as was Charles Dowd's original proposal, submitted in 1870, which proposed the installation of four, the first of which passed through Washington. In 1872, Dowd revised his proposal, changing the center of reference to Greenwich Mean Time. It was this last proposal of his, almost unchanged, that was used by the railroads of the United States of America and Canada eleven years later.

November 18, 1883 The American and Canadian railroads changed their clocks for all railway stations according to (forward or backward). The belts were named East, Central, Mountain and Pacific.

Despite the transition of major railroads in the United States and Canada to standard time, it was still many years before the latter became the norm in everyday life. But the use of standard time began to spread rapidly, given its obvious practical benefits for communication and travel.

During the year, 85% of all cities North America(about 200), whose population exceeded 10,000, were already using standard time. Only Detroit and Michigan stood out noticeably.

Detroit lived on local time until 1900, when the City Council issued a decree requiring the clock to be set back twenty-eight minutes to Central Standard Time. Half the city complied, and half refused. After considerable debate, the decree was rescinded and the city returned to solar time. In 1905, Central Time was adopted by city vote. By a city ordinance in 1915, and then by a vote in 1916, Detroit switched to Eastern Standard Time (EST).

Throughout the United States, normal time was introduced with the passage of the Standard Time Act of 1918. The US Congress approved the standards set by the railroads earlier, and transferred responsibility for any subsequent changes to them to the Interstate Commerce Commission, and at that time the only federal body for regulating transportation. In 1966, the authority to pass legislation relating to the definition of time was transferred to the Department of Transportation created under Congress.

The existing borders in the United States today have been significantly changed compared to their original version, and such changes are still taking place. The Department of Transportation handles all change requests and conducts rulemaking. In general, the borders tend to move westward. For example, at the eastern end, sunset may be replaced by an hour later (hourly) by a transition to a time zone adjacent to the east. Thus, the boundaries of the time zone are locally shifted to the west. The reasons for this phenomenon are similar to the reasons for the introduction of "maternity" time in Russia (see Summer time). The accumulation of such changes leads to a long-term trend of movement of the boundaries of the belts to the west. This is not unstoppable, but is very undesirable, since it also entails a late sunrise in such areas, especially in winter. According to US law, the main factor in deciding whether to change the time zone is "facilitating business." According to this criterion, the proposed changes were both approved and rejected, but most of them were accepted.

What is Unix time or Unix epoch (Unix epoch or Unix time or POSIX time or Unix timestamp) ?

UNIX time or POSIX time (eng. Unix time) - a way of encoding time adopted in UNIX and other POSIX-compatible operating systems.
The starting point is midnight (UTC) from December 31, 1969 to January 1, 1970, the time from that moment is called the "UNIX era" (Eng. Unix Epoch).
UNIX time is consistent with UTC, in particular, when declaring UTC leap seconds, the corresponding second numbers are repeated.
The way of storing time as a number of seconds is very convenient to use when comparing dates (accurate to the second), and also for storing dates: if necessary, they can be converted to any human-readable format. Date and time in this format also take up very little space (4 or 8 bytes, depending on the size of the machine word), so it is reasonable to use it for storing large amounts of dates. The performance disadvantages can show up when accessing date elements very frequently, such as the number of the month, etc. But in most cases it is more efficient to store the time as a single value, rather than a set of fields.

Convert Unix epoch to human readable date

Unix start and end date of year, month or day

Convert seconds to days, hours and minutes

How to get Unix time in...

Convert date to Unix time to...

Converting Unix time to human readable date...

What is the "Unixtime converter" tool for?

This tool, first of all, will be useful for webmasters who constantly deal with large volumes of dates or often refer to their elements in their work. Using the "Unixtime converter" tool, you can easily convert Unix time to a user-friendly date (and vice versa), find out the current Unix epoch time, and get Unix time in various programming languages, DBMS and operating systems.

What is Unix Time?

The Unix era (Unix epoch) began on the night of December 31, 1969 to January 1, 1970. It was this date that was taken as the starting point of "computer" time, which is calculated in seconds and takes up very little disk space - only 4 or 8 bytes. With this encoding method, programmers can "hide" any date into a single number, and easily convert it back into a user-friendly format.

Unix time (also called Unix time or POSIX time) is convenient to use in various operating systems and programming languages, since it is displayed as a single value, and not a certain number of fields that take up space. In addition, UNIX time is fully consistent with the UTC standard (including in leap years) - in this case, the corresponding seconds simply repeat.

Unix terminology

A few words about terms.

So, Unix time(or POSIX time) is the number of seconds that have elapsed since midnight, January 1, 1970, to the present.

Unix Timestamp(timestamp) is a "fixed" time, in other words, a specific date imprinted in a number.

UTC(Universal Coordinated Time) is the Universal Coordinated Time, which is "fixed" on the zero meridian, and from which geographical time zones are counted.

How durable is this system?

In just a couple of decades, namely on January 19, 2038 at 03:14:08 UTC, Unix time will reach the value 2147483648, and computer systems can interpret this number as negative. The key to solving this problem lies in using a 64-bit (instead of 32-bit) variable to store the time. In this case, the stock of numerical values ​​of Unix time will be enough for humanity for another 292 billion years. Not bad, right?

Unix time is the same for everyone

If you live in London or San Francisco, and your friends are in Moscow, then you can "synchronize watches" using Unix time: this system is in this moment time is the same for the whole world. Naturally, if the time on the servers is set correctly. And with the tool "unixtime converter" such a conversion will take you a fraction of a second.

Standard time - a system of counting hourly time, based on the division of the Earth's surface into 24 time zones, every 15 ° in longitude. Time within the same time zone is considered the same. In 1884, on International Conference decision was made to use this system. In accordance with the international agreement of 1883, the initial ("zero") meridian is the one that passes through the Greenwich Observatory in the suburbs of London. Local Greenwich time (GMT), agreed to be called universal or "World time"

On the territory of Russia, since March 28, 2010 - 9 time zones (before that there were 11 time zones). Samara Region and Udmurtia switched to Moscow time (second time zone). Kemerovo region. (Kuzbass) - to Omsk (MCK+3). Kamchatka Territory and Chukotka - to Magadan (MSK + 8). In these five subjects of the Federation, March 28, 2010 - the clock hands were not translated.

Two belts are abolished - the third (Samara, MSK + 1) and the eleventh (Kamchatsky, MSK + 9). In total, there are 9 of them, and the maximum spread in time in our country is reduced from 10 to 9 hours.

In Russia, since March 2011, after the transition to summer time, the clock hands are no longer translated during the year.

In 2012 - again discussed, at all levels, the advantages of a permanent winter time over summer time, therefore - a transition (this autumn) to a permanent, year-round winter time.

Stable time is better for health. In the autumn-spring off-season, the body will not have to especially rebuild its biorhythms. Technical services and transport workers no longer have to, as before, when moving the hands of the clock, reconfigure equipment and change schedules.

Moscow time zone, stable time: +4 (GMT + 4:00)

The boundaries of standard time are drawn taking into account physical and geographical features - according to big rivers, watersheds, as well as along interstate and administrative borders. States can change these boundaries within the country.

Applies international system U T C (World time; it is denoted by UTC / GMT or, which is the same thing - UTC), as well as the difference between local and Moscow time - MSK. The plus sign means - to the east, "minus" - to the west of the starting point

The transition to summer time (one hour ahead) and winter time (one hour ago) is carried out in spring and autumn, respectively. This rule is valid in the European Union, Egypt, Turkey, New Zealand... Dates and order of transfer - may slightly differ in terms. Most countries have abandoned the autumn-spring shift of the clock: Russia and Belarus (since 2011), Kazakhstan, Turkmenistan, Uzbekistan, Tajikistan, India, China, Japan, Singapore, Taiwan ...

World time - UTC/GMT - the value of Greenwich Mean Time (G M T) is equal to "Universal Coordinated Time" (U T C) with an accuracy of one second - GMT=UTC). The name U T C will, over time, completely replace the term "Greenwich time"

Rice. 2 Map - World time zones and their offsets from UTC / GMT (Greenwich Mean Time)

Table - time zones of world cities (UTC / GMT), in the summer

An example of the designation of winter and summer time: EST / EDT (Eastern Standard / Daylight Time Zone).
If, somewhere, winter time is considered standard, then it can be abbreviated, for example: ET, CT, MT, PT

Table - time zones of cities and regions in Russia, since 2011.
The local time difference is shown:
MSK + 3 - with Moscow;
UTC+7 - with Coordinated Universal Time (UTC = GMT)

Note: MSK = MSD (Moscow Summer Time) throughout the year

Terms and Definitions

Switching to Daylight Saving Time (DST - Daylight Saving (Summer) Time) - moving the clock forward one hour, which was carried out annually on the last Sunday of March, in order to get an extra hour during daylight hours, to save electricity (for lighting, etc. ). The return to winter time was carried out in the last. Sunday October. These transitions affected the biorhythms of the human body, his well-being, and it took a week of adaptation to get used to. Manipulation of the hands of the clock is a common cause of lateness of workers and employees to work.

The initial (zero) meridian is the Greenwich meridian with geographic longitude equal to 0°00"00", which divides the globe into the western and eastern hemispheres. Passes through the former Greenwich Observatory (in the suburbs of London)

GMT (Greenwich Mean Time) - "Greenwich Mean Time" on the Greenwich meridian. It is determined by astronomical observations of the daily movement of stars. It is unstable (within a second per year) and depends on the constant change in the speed of the Earth's rotation, the movement of the geographic poles on its surface and the nutation of the planet's axis of rotation. Greenwich (astronomical) time is close in value to UTC (atomic time), and for the time being will be used as its synonym. Also called "Zulu Time"

In Russian-speaking meteorology, GMT is denoted as GMT (Greenwich Mean / or Geographic / Time)

GMT= UTC (accurate to 1 second)

Time zone (Standard time zone) - difference from UTC / GMT World Time (example: UTC / GMT + 4 - the fourth time zone, east of Greenwich)

H:mm:ss - 24 hour format (example: 14:25:05). Minutes and seconds - with leading zeros

h:mm:ss - 12-hour format (example: 02:25:05 PM - "two and a half hours in the afternoon" - 14:25:05). Minutes and seconds - with leading zeros

AM - designation of time before noon in 12-hour format (abbreviated version - "A")
PM - designation of time after noon in 12-hour format

Universal Time UT (Universal Time - Universal Time) - the mean solar time on the Greenwich meridian, is determined by astronomical observations of the daily movements of stars. Its updated values ​​are UT0, UT1, UT2

UT0 - time on the instantaneous Greenwich meridian, determined by the instantaneous position of the Earth's poles

UT1 - time on the Greenwich mean meridian, corrected for the movement of the earth's poles

UT2 - time, taking into account changes in the speed of rotation of the Earth

TAI - atomic clock time (International Atomic Time, since 1972). Stable, reference, never translated. Time and frequency standard

The time in the GPS navigation system has been valid since January 1980. It does not include amendments. It is ahead of the UTC time by 15 seconds.

UTC (from English Universal Time Coordinated)- Coordinated Universal Time for the coordinated distribution of standard frequencies and accurate time signals on radio, television and the Internet - "World Time". Its synonym is "Universal Time Zone"

The UTC time scale has been introduced since 1964 to harmonize UT1 (astronomical measurements) and TAI (atomic clocks).

Unlike GMT, UTC is set using atomic clocks.

The speed of rotation of the earth is slowing down, in connection with which the UTC scale is regularly, after a year or two or three, on June 30 or December 31, amendments are introduced (leap seconds - "Second of coordination"), so that U T C is no more than a second ( more precisely - 0.9 s) differed from astronomical time (determined by the movement of the Sun), as UT1 lagged behind by a second. This international rule was adopted in 1972.

The ratio of time in 2009: UTC (universal) lags behind TAI (atomic) - by 35s. Time in the GPS navigation system - ahead of UTC by 15 seconds (the countdown is from 1980, the difference is increasing) T glonass = Tutc + 3 hours (adjusted, so the difference between them does not exceed 1 ms.)

Precise time signals (for clock synchronization) are transmitted via radio channels, television, the Internet - in the UTC system. More precisely, you can put, for example, on the signal of the Mayak radio, but only on the long-wave or medium-wave range (on the "earth-surface wave"). On the VHF / FM radio band, the signal may be delayed up to several seconds from the true one.

In watches with automatic synchronization (Eng. Radio controlled), time correction occurs from base stations, on ultra-long waves. This system has been developed in Europe.

Numbers of exact local time services in Russian cities 100 - Moscow Voronezh Cheboksary Chelyabinsk 060 - Bryansk Kaliningrad Krasnodar Murmansk St. Petersburg Samara Cellular operators do not have such a service, since the mobile phone is not limited geographically and can work not only in a certain city, but also in roaming .

UTC time is not translated either in winter or summer, therefore, for those places where there is a transfer to summer time, the offset relative to UTC changes (in Moscow, before the abolition of winter time in 2011, the difference was: in winter - UTC + 3, in summer - UTC+4).

Standard abbreviations calendar months and days of the week in English (applied in RSS and others): January February March April May June July August September October November December Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Monday Tuesday Wednesday Thursday Friday Saturday Sunday Mon Tue Wed Thu Fri Sat Sun

GMT - Greenwich Mean Time (GMT) - the time of the meridian passing through the old Greenwich Observatory near London. It is used as a designation of time on weather charts. Synonyms for GMT are GMT and UTC.

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Literature

"Time and calendar" - M .: Nauka. 1989

Global (satellite) navigation systems GLONASS (Russia), GPS (USA), Galileo (European Union) - make it possible to determine, using navigators, including portable ones, the current location (coordinates), trajectory and speed of objects in any point of our planet and in near-Earth outer space.

According to the method of operation and purpose, satellite GPS (Global Positioning System) navigators are automobile (car navigators), portable, marine, etc. The most common imported ones are Garmin, Mio, etc. There are completely autonomous configuration options - with battery charging from solar panels or miniature thermoelectric generators (thermocouples). The navigation system is built into modern communicators, smartphones and Cell Phones, allowing you to get not only the geographic coordinates of the receiver's location, but also the system time with an accuracy of fractions of a microsecond.

The Russian GLONASS (Glonass) has been operating since the mid-1990s. The orbital constellation includes more than two dozen active satellites. The system operates throughout Russia. Since 2009, transport, including passenger vehicles, has been massively equipped with this system.

Navigators are produced in Russia (Glospace SGK-70 and others) that can work simultaneously with several navigation systems - GLONASS, GPS, Galileo.

Glospace supports the SMILINK system (shows traffic jams) and is able to lay detour routes. Signals can be received from several satellite systems at once.

G P S maps - electronic maps for navigators and other mobile devices (communicators, pda / pda, smartphones, etc.) with GPS function.

• UTC: Time at the prime meridian is called Universal Coordinated Time. The mismatch of the acronym was caused by the need for its universality for all languages.
• GMT: Previously, Greenwich Mean Time (GMT) was used instead of UTC, since the prime meridian was chosen to pass through the Greenwich Royal Observatory.
• Other time zones may be written as an offset from UTC. For example, Australian Eastern Standard Time (EST) is written as UTC+1000, meaning 10:00 UTC is 20:00 EST on the same day.
• Summer time does not affect UTC. This is just a political decision to change the time zone (offset from UTC). For example, GMT is still in use: it is British national time in winter. In summer it becomes BST.
• Leap seconds: By international convention, UTC is kept no more than 0.9 seconds from physical reality (UT1, which is measured in solar time) by introducing a "leap second" at the end of the last minute of the UTC year or the last minute of June.
• Leap seconds are not required to be declared (by astronomers) more than 6 months before they are introduced. This is a problem if you need any second-to-second planning for more than 6 months.
• Unix Time: Measured by the number of seconds since the "epoch" (beginning of 1970 UTC). Unix time is not affected by time zones or daylight saving time.
• According to the POSIX.1 standard, Unix time is supposed to handle a leap second by repeating the previous second, for example: 59.00 59.25 59.50 59.75 59.00 ← repeat 59.25 59.50 59.75 00.00 ← increment 00.25 This is a compromise: you can't express a leap second in your system clock and your time is guaranteed to go backwards. On the other hand, each day is exactly 86,400 seconds, and you don't need a table of all past and future leap seconds to convert Unix time into human-friendly hours-minutes-seconds.
• ntpd is supposed to retry after it receives "leap bits" from upstream time servers, but I've also seen it do nothing: the system jumps one second into the future, then slowly slides back to the correct time.

What every programmer should know about time

• Time zones refer to presentation layer
  Most of your code should not deal with timezones or local time, it must pass the Unix time as it is.
• When measuring time, measure Unix time. This is UTC. It's easy to get (by system functions). It has no time zones or daylight saving time (and leap seconds).
• When storing time, store Unix time. This is one number.
• If you want to save human-readable time (e.g. in logs), try to save it together with Unix time, not instead of.
• When displaying the time, always include the time zone offset. The time format without offset is useless.
• The system clock is not accurate.
• You are online? Every other machine's system clock is not accurate in different ways.
• The system clock can, and will, jump back and forth in time due to things that are out of your control. Your program must be designed to survive this.
• The ratio of the number of seconds system clock to quantity real seconds is not exact and may vary. It mainly depends on the temperature.
• Don't blindly use gettimeofday() . If you need monotonic (constantly increasing) clocks, look at clock_gettime() . [Java option: Use System.nanoTime() instead of System.currentTimeMillis() ]
• ntpd can change the system time in two ways:
  • Step: the clock jumps forward or backward to the correct time immediately
  • Twist: Change the frequency of the system clock so that it moves slowly towards the correct time.
  Twisting is preferred because it is less destructive, but only useful for correcting a small difference.

Special occasions

• Time passes at a rate of one second per second for all observers. The frequency of remote clocks relative to the observer depends on the speed and gravity. Clocks inside GPS satellites are adjusted to overcome the effects of relativity.
• MySQL stores DATETIME columns as "YYYYMMDD HHMMSS" values ​​packed into numbers. If you care to store timestamps, store them as an integer and use the UNIX_TIMESTAMP() and FROM_UNIXTIME() functions to convert.