I often see various terms: GMT, time zone, daylight saving time, etc. This article briefly introduces the relevant knowledge.
Greenwich Mean Time (GMT)
Also known as Greenwich Mean Time (GMT), it refers to the local standard time at the Royal Greenwich Observatory on the outskirts of London, England, because the prime meridian is defined as the longitude passing there.
In theory, noon on GMT is the time when the sun crosses the Greenwich meridian (that is, when it is at its highest point above Greenwich). However, due to the uneven speed of the earth in its elliptical orbit, this moment may be wrong with the actual solar time, with a maximum error of 16 minutes. The reason is that the daily rotation of the earth is somewhat irregular and is slowly decelerating, so GMT is no longer used as a standard time based on the shortcomings of astronomical observations themselves. The current standard time is determined by the Coordinated Universal Time (UTC), which is reported by atomic clocks.
Coordinated Universal Time (UTC)
Coordinated Universal Time (English: Coordinated Universal Time, French: Temps Universel Coordonné, UTC for short) is the main world time standard, which is based on atomic time seconds and is as close to Greenwich Mean Time as possible.
Coordinated Universal Time is one of several alternative time systems that are closest to Greenwich Mean Time (GMT). For most uses, UTC time is considered interchangeable with GMT time, but GMT time is no longer determined by the scientific community.
Time zone
A time zone is an area on Earth that uses the same time definition. Previously, people determined time by looking at the position of the sun (hour angle), which made the time at different longitudes different (local time). In 1863, the concept of time zones was first used. Time zones partially solve this problem by establishing a zone's standard time.
Every country in the world is located in different positions on the earth, so different countries, especially those with a large east-west span, must have some deviations in the sunrise and sunset times. These deviations are known as jet lag.
time zone notation
If the time is in Coordinated Universal Time (UTC), add a "Z" (without a space) directly after the time. "Z" is the sign for time zone 0 in Coordinated Universal Time. Therefore, "09:30 UTC" is written as "09:30Z" or "0930Z". "14:45:15 UTC" is "14:45:15Z" or "144515Z".
UTC time is also called Zulu time, because "Zulu" is used for "Z" in the NATO phonetic alphabet.
UTC offset
UTC offset is the difference between Coordinated Universal Time (UTC) and a date and time at a specific location, in hours and minutes. It is usually displayed in the format ±[hh]:[mm], ±[hh][mm], or ±[hh]. So, if the time being described is one hour ahead of UTC (eg winter time in Berlin), the offset from UTC will be "+01:00", "+0100", or simply "+01".
Beijing time
Beijing time, also known as China Standard Time, is the standard time in mainland China, which is eight hours ahead of Coordinated Universal Time (ie UTC+8), and is the same as the standard time in Hong Kong, Macau, Taipei, Kuala Lumpur, Singapore and other places.
The following figure shows the time zone in which each country is located. Note that a country may span multiple time zones.
For example, France is the country with the most time zones, with a total of 12 time zones.
time zone information database
We often see such a time zone representation: "Asia/Shanghai", and this information is stored in the time zone information database.
Time zone information database, also known as TZ database, Zoneinfo database, is a database mainly used in computer programs and operating systems, which can collaboratively edit world time zone information. Since the database was created by David Olson, it is also referred to in some places as the Olson database. The database is edited and maintained by Paul Eggert.
Its distinguishing feature is a set of common time zone naming conventions devised by the aforementioned Paul Eggert, such as "America/New_York" and "Europe/Paris".
To make it easier for us to understand, each time zone gets a unique name in the format "area/location", such as "America/New_York". The spaces in English place names are replaced by underscores "_", and the hyphen "-" is only used when the English place name itself is included.
daylight saving time
Daylight saving time, also translated as summer time, also known as daylight saving time, daylight saving time (English: Daylight saving time), is a system that artificially regulates local time in order to save energy. The unified time used during the implementation of the system is called "daylight saving time". Generally, in the summer when the dawn is earlier, the time is artificially adjusted by one hour, which can make people get up early and go to bed early, reduce the amount of lighting, and make full use of light resources, thereby saving lighting electricity. Different countries have different rules for adopting daylight saving time.
Judging from the data, most countries and regions do not implement daylight saving time.