1. Data structure
time_t integer; 64-bit integer under linux, 64-bit integer under windows, you can open the macro _USE_32BIT_TIME_T to force interpretation as 32-bit integer.
struct tm {
int tm_sec; /* seconds after the minute - [0,59] */
int tm_min; /* minutes after the hour - [0,59] */
int tm_hour; /* hours since midnight - [0,23] */
int tm_mday; /* day of the month - [1,31] */
int tm_mon; /* months since January - [0,11] */
int tm_year; /* years since 1900 */
int tm_wday; /* days since Sunday - [0,6] */
int tm_yday; /* days since January 1 - [0,365] */
int tm_isdst; /* daylight savings time flag */
};//Note that tm_mon tm_year tm_wday tm_yday is not from the beginning, pay attention to processing
2. Conversion
time => time_t (UTC)
gmtime; time_t(UTC)=>tm The upper limit of time is 2038 or 3000 (depending on time_t 32-bit or 64-bit)
mktime; tm(calendar)=>time_t
_mkgmtime; tm(UTC)=>time_t
localtime;time_t(UTC)=>tm(calendar)
//These functions are not thread-safe, due to shared tm, coverage problems may occur
reference：
https://docs.microsoft.com/zh-cn/previous-versions/1f4c8f33(v=vs.110)?redirectedfrom=MSDN
https://docs.microsoft.com/zh-cn/previous-versions/w4ddyt9h%28v%3dvs.120%29