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I don't understand time, timestamp and time zone. Look at this article

2022-04-23 08:58:00 【InfoQ】

UTC  Coordinated universal time , Also known as universal time 、 World standard time 、 International coordination time . There are other time zones , for example ： Beijing time. 、 The United States time .

Unix  The time stamp is from  1970-1-1 00:00:00 UTC （ Beijing time. 1970-01-01 08:00:00） The number of seconds to start , Leap seconds not considered , also called  Unix epoch、Unix time、POSIX time.

At a specific moment , There is only one timestamp , for example , I just read the timestamp once , The value is 1567819100. At that moment , Users in China 、 North Korea 、 Pakistan 、 The values read by the United States are the same , Because the timestamp is relative to UTC The time of the .

The time zone is different , This moment can be expressed as a lot of time , As shown in the figure below ：

C The language provides timestamp types and time types , It also provides functions for the conversion between the two , As shown below ：

typedef long time_t;
struct tm {
 int tm_sec;
 int tm_min;
 int tm_hour;
 int tm_mday;
 int tm_mon;
 int tm_year;
 int tm_wday;
 int tm_yday;
 int tm_isdst;
};

struct tm *gmtime(time_t const *time_value); //  Time stamping  UTC  Time
char *setlocale(int category, char const *locale); //  Set up Local
struct tm *localtime(time_t const *time_value); //  Time stamp to local time

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