Daylight saving time in the context of Iran Standard Time

There are 11 time zones in Russia, which currently observe times ranging from UTC+02:00 to UTC+12:00. Daylight saving time (DST) has not been used in Russia since 26 October 2014. From 27 March 2011 to 26 October 2014, permanent DST was used.

A time zone is an area which observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it is convenient for areas in frequent communication to keep the same time.

Each time zone is defined by a standard offset from Coordinated Universal Time (UTC). The offsets range from UTC−12:00 to UTC+14:00, and are usually a whole number of hours, but a few zones are offset by an additional 30 or 45 minutes, such as in India and Nepal. Some areas in a time zone may use a different offset for part of the year, typically one hour ahead during spring and summer, a practice known as daylight saving time (DST).

Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a consequence, it cannot be used to specify a particular time unless a context is given. The term "GMT" is also used as one of the names for the time zone UTC+00:00 and, in UK law, is the basis for civil time in the United Kingdom.

Because of Earth's uneven angular velocity in its elliptical orbit and its axial tilt, noon (12:00:00) GMT is rarely the exact moment the Sun crosses the Greenwich Meridian and reaches its highest point in the sky there. This event may occur up to 16 minutes before or after noon GMT, a discrepancy described by the equation of time. Noon GMT is the annual average (the arithmetic mean) moment of this event, which accounts for the word "mean" in "Greenwich Mean Time".

In modern usage, civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally national standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972. Older systems use telescope observations.

In traditional astronomical usage, civil time was mean solar time reckoned from midnight. Before 1925, the astronomical time 00:00:00 meant noon, twelve hours after the civil time 00:00:00 which meant midnight. HM Nautical Almanac Office in the United Kingdom used Greenwich Mean Time (GMT) for both conventions, leading to ambiguity, whereas the Nautical Almanac Office at the United States Naval Observatory used GMT for the pre-1925 convention and Greenwich Civil Time (GCT) for the post-1924 convention until 1952. In 1928, the International Astronomical Union introduced the term Universal Time for GMT beginning at midnight.

Local time is the time observed in a specific locality. There is no canonical definition. Originally it was mean solar time, but since the introduction of time zones it is generally the time as determined by the time zone in effect, with daylight saving time where and when applicable. In some places this is known as standard time.

Some sources continue to use the term local time to mean solar time as opposed to standard time, but they are in the minority. Terms such as local mean time also relate to solar time.

South Korea has one time zone, Korea Standard Time (UTC+9), which is abbreviated KST. South Korea currently does not observe daylight saving time.

From May 8 to October 9 in 1988, daylight saving time was tested to better accommodate the calendar of competitions held during the 1988 Summer Olympics in Seoul.

The time in China follows a single standard time offset of UTC+08:00, where Beijing is located, even though the country spans five geographical time zones. It is the largest sovereign nation in the world that officially observes only one time zone.

The nationwide standardized time is named Beijing Time (BJT; Chinese: 北京时间) domestically and China Standard Time (CST) internationally. Daylight saving time has not been observed since 1991.

Kaliningrad Time (KALT; Russian: калининградское время, romanized: kaliningradskoye vremya) is the time zone two hours ahead of UTC (UTC+02:00) and one hour behind Moscow Time (MSK−1). It is used in Kaliningrad Oblast.

Until 2011, Kaliningrad Time was identical to Eastern European Time (UTC+02:00; UTC+03:00 with daylight saving time). On 27 March 2011, Russia moved to permanent DST, switching Kaliningrad time permanently to UTC+03:00. On 26 October 2014, this law was reversed but daylight saving time was not reintroduced, so Kaliningrad is now permanently set to UTC+02:00.

Moscow Time (MSK; Russian: моско́вское вре́мя, romanized: moskovskoye vremya) is the time zone for the city of Moscow, Russia, and most of western Russia, including Saint Petersburg. It is the second-westernmost of the eleven time zones of Russia, after the non-continguous Kaliningrad enclave. It has been set to UTC+03:00 without DST since 26 October 2014; before that date it had been set to UTC+04:00 year-round on 27 March 2011.

Moscow Time is used to schedule trains (until 1 August 2018), ships, etc. throughout Russia, but air transport in Russia is scheduled using local time. Since 1 August 2018, Russian railways switched to using local time. Time in Russia is often announced throughout the country's other timezones on radio stations as Moscow Time, which is also registered in telegrams, etc. Descriptions of time zones in Russia are often based on Moscow Time rather than UTC; for example, Yakutsk (UTC+09:00) is said to be MSK+6 in Russia.

Yekaterinburg Time (YEKT) is the time zone five hours ahead of UTC (UTC+05:00) and 2 hours ahead of Moscow Time (MSK+2).

In 2011, Russia moved to year-round daylight saving time. Instead of switching between UTC+05:00 in winter and UTC+06:00 in summer, Yekaterinburg time was set to UTC+06:00 until 2014, when it was reset back to UTC+05:00 year-round.

Vladivostok Time (VLAT) (Russian: владивостокское время, vladivostokskoye vremya), is a time zone in Russia, named after the city of Vladivostok. It is ten hours ahead of UTC (UTC+10:00) and seven hours ahead of Moscow Time (MSK+7).

On 27 March 2011, Russia moved to year-round daylight saving time. Instead of switching between UTC+10:00 in winter and UTC+11:00 in summer, Vladivostok Time became fixed at UTC+11:00 until 2014, when it was reset back to UTC+10:00 year-round.

Central European Summer Time (CEST, UTC+02:00), sometimes referred to as Central European Daylight Time (CEDT), is the standard clock time observed during the period of summer daylight-saving in those European countries which observe Central European Time (CET; UTC+01:00) during the other part of the year. It corresponds to UTC+02:00, which makes it the same as Eastern European Time, Central Africa Time, South African Standard Time, Egypt Standard Time and Kaliningrad Time in Russia.

Australia uses three main time zones: Australian Eastern Standard Time (AEST; UTC+10:00), Australian Central Standard Time (ACST; UTC+09:30) and Australian Western Standard Time (AWST; UTC+08:00).

Time is regulated by the individual state governments, some of which observe daylight saving time (DST). Daylight saving time (+1 hour) is used between the first Sunday in October and the first Sunday in April in jurisdictions in the south and south-east:

Eastern European Time (EET) is one of the names of the UTC+02:00 time zone, 2 hours ahead of Coordinated Universal Time. The zone uses daylight saving time, so that it uses UTC+03:00 during the summer.

A number of African countries use UTC+02:00 all year long, where it is called Central Africa Time (CAT), although Egypt and Libya also use the term Eastern European Time.

Midnight is the transition time from one day to the next – the moment when the date changes, on the local official clock time for any particular jurisdiction. By clock time, midnight is the opposite of noon, differing from it by 12 hours.

Solar midnight is the time opposite to solar noon, when the Sun is closest to the nadir, and the night is equidistant from sunset and sunrise. Due to the advent of time zones, which regularize time across a range of meridians, and daylight saving time, solar midnight rarely coincides with 12 midnight on the clock. Solar midnight depends on longitude and time of the year rather than on time zone. In ancient Roman timekeeping, midnight was halfway between dusk and dawn (i.e., solar midnight), varying according to the seasons.

Time in North Korea, called Pyongyang Time (PYT; Korean: 평양시간) or Standard Time of the Democratic People's Republic of Korea (조선민주주의인민공화국 표준시간; 朝鮮民主主義人民共和國標準時間), is since May 2018 equal to Korea Standard Time: 9 hours ahead of UTC (UTC+09:00). Like South Korea, North Korea does not currently observe daylight saving time. Time keeping in North Korea is under the State Commission for Science and Technology.

The Republic of Indonesia, a country located in Southeast Asia has three time zones. Western Indonesia Time (Waktu Indonesia Barat, WIB) is seven hours ahead (UTC+07:00) of the Coordinated Universal Time (UTC), used in the islands of Sumatra, Java, and the western half of Kalimantan. Central Indonesia Time (Waktu Indonesia Tengah, WITA) is eight hours ahead (UTC+08:00), used in the eastern half of Kalimantan, as well as all of Bali, the Lesser Sunda Islands, and Sulawesi. Eastern Indonesia Time (Waktu Indonesia Timur, WIT) is nine hours ahead (UTC+09:00), used in the Maluku Islands and Western New Guinea. About 80% of the Indonesian population live in WIB (UTC+07:00).

In 1908, during the Dutch East Indies colonial era, only Java and the Madura Island were initially given time until 1932, when the government utilised UTC+06:30 up to +09:30. In between those changes in 1918, Central Java (UTC+07:20, now defunct) was the basis for time in select locations: for instance, Padang was 7 minutes behind Central Java. The Japanese occupation of the Indies prompted the simplification of time in Indonesia to just UTC+09:00, also known as the Japan Standard Time. The Dutch reoccupied the country following its 1945 independence with UTC+06:00 up to +09:00 established, but it was reverted to the 1932 system after they recognised Indonesia's sovereignty. The current time zone division is a 1988 revision of a 1964 division, with Bali, West Kalimantan, and Central Kalimantan changing sides. Without any significant season changes, the daylight saving time system is not observed anywhere in Indonesia, as is other Southeast Asian countries.

Timor-Leste uses UTC+09:00. In the west the country borders the UTC+08:00 zone of Central Indonesia and in the east the UTC+09:00 zone of Eastern Indonesia. Daylight saving time is never observed in Timor-Leste, due to its proximity to the equator; there is only a small variation between the length of day and night throughout the year. Timor-Leste shares the same time zone with Japan Standard Time, Korean Standard Time, Pyongyang Time (North Korea), Eastern Indonesia Standard Time, Palau Time, and Yakutsk Time (Russia).