A second is a basic unit of time. 24 hours a day, 60 minutes in a hour, 60 minutes in a minute, which comes from the definition of seconds – the interval of one second is 1  86400 of the average solar day. By the middle of twentieth Century, it was discovered that the rotation time of the earth was not constant, so it was at 195.In the 6 year, we define the second  as the earth revolves around the sun orbit for a week. In 1967, the atomic resonance frequency was used to define the second , and on this basis, the atomic clock was produced. At present, atomic clocks are the most accurate time measurement and frequency standards in the world.
Greenwich peacetime (Greenwich Mean Time, GMT) is also known as Greenwich standard time.
The normal noon in Greenwich is the time when the flat sun crosses the meridian of Greenwich (that is, the highest point over Greenwich). Since February 5, 1924, the Greenwich Observatory is responsible for distributing time messages to the world every hour. Because the earth’s rotation is irregular.And it is slowing down slowly, so Greenwich is usually replaced by the coordinated time of the clock (UTC) based on the astronomical observation itself.
In the HTTP protocol, all time is converted to GMT time , for example.
date: Sat, 19 May 2018 17:17:24 GMT
Universal Time (UT) is a kind of flat sun at the beginning of midnight in Greenwich.
Since 1925, in astronomical observations, the beginning of the day (0) was often fixed at noon, rather than the usual civil midnight, which was vague for the meaning of Greenwich’s ordinary meaning.
Time zone refers to the definition of a certain area on the earth using the same time. GMT time or UT time is a form of the rotation rate of the earth. From the rising of the sun to the sun, it changes from 0 to 24. In this way, the local time of different longitude will naturally be different. In order to solve this problem,The earth is divided into different regions according to longitude. Each region uses the same time definition. The time difference between adjacent regions is 1 hours. The time zone is divided into the theoretical time zone and the legal time zone.
theoretical time zone”
According to longitude, every 15 degrees is a time zone. The earth is divided into 24 time zones, with the primary meridian as the center, and the area extending 7.5 degrees to the East and west sides is the 0 time zone.
legal time zone
The legal time zone is based on the theoretical time zone and adjusted according to the national boundaries of some regions. A time zone for actual use. For example, China crosses five time zones across the East five District to the East nine district, but uses East eight district time uniformly (Beijing time).
The difference between the time at one place and the time in the 0 time zone is called the time difference. Taking the primary meridian as the center, every time we go to the east one across a time zone, it increases by one hour, and every time we go west across a time zone, it decreases by one hour.
How to understand the increase to the east time zone
As the earth moves from west to East, it is observed in a certain part of the earth, and the eastern time is earlier than that in the West (the people in the East first see the sun rise).
Imagine a time when the sun is above you, where you are at 12 noon. At this time, the people living in your east side see that the sun is already in the West. Their time is in the afternoon.
international date change line
In general, the 180 degree meridian is the boundary line. When crossing the date line from west to East, the date needs to be reduced by one day, on the contrary, the date is increased by one day.
When coordinating the world (Coordinated Universal Time). It is the main world time standard. It is based on the second time defined by atomic clocks and is as close as possible to GMT time at any time. UTC time thinks that a solar day is always 86400 seconds. In most casesAt the same time, UTC time can be interchanged with GMT time.
UTC and time zone
Characters are added to the time zone in which the primary meridian is located.
Z，The UTC time is represented.
ZThat is the sign of the 0 time zone, read and do
09:30 UTCOn writing
UTCThe offset is expressed in the following form:
±[hh]。For example, UTC time is
09:30z，At this time Beijing time is
1730 +0800，New York time is
UTCThe format of the time representation is generally
Sat, 20 May 2018 12:45:57 +0800It means East eight district (Beijing time) May 20, 2018 12:45:57 Saturday.
UTC time and local time conversion”.
UTCTime + time difference = local time
2018-05-20 09:30:00 UTC，UTC+0800(Beijing)
2018-05-20 17:30:00 +8000，UTC-0500(New York)
2018-05-20 04:30:00 -0500
2018-05-20 01:30:00 UTC，UTC+0530(India)
2018-05-20 07:00:00 +0530，UTC-1000(Hawaii)
2018-05-19 15:30:00 -1000
UNIXTime, or POSIX time, is the time expression used by UNIX or UNIX like systems. It is generally defined as the total number of seconds from the coordinated world time (UTC time) from 0 hours, 0 minutes and 0 seconds in January 1, 1970 to the present. Considering the leap seconds, it is more precisely defined from the coordinated universal time (U).TC time) the total number of seconds after adjusting 0 seconds, 0 minutes and 0 seconds in January 1, 1970.
UNIX time stamp and leap second”
Because of the existence of leap seconds, when a modified leap second occurs, a Unix timestamp may indicate two moments. However, there is no regularity in the adjustment of leap seconds, which leads to inaccuracy in calculating the time interval with UNIX timestamp. The computer passes the network time protocol (Network Ti)Me Protocol (NTP) to receive leap seconds adjusted messages.
This blog explains the problem and gives an interesting suggestion link.
 The average solar day is the unit of time based on observation and artificial adjustment. An average solar day is fixed for 24 hours.
 1960. From ephemeris to 12 hours in January 1, 1900, 31556925.9747 of the year of the regression year is one second.
 1The second cycle of the two hyperfine energy transitions for the ground state of cesium 133 corresponds to the 9192631770 cycle duration of the radiation.
 The international atomic time standard is a continuous time scale, which is based on the seconds defined by the atomic clock, and is calculated by day, time, minute and second from 0 hours, 0 minutes and 0 seconds in January 1, 1958. The accuracy of the atomic time scale is nanoseconds per day, while the accuracy of the world time is only a few milliseconds.