Linux系统UTC和CST时间转换

        在Linux系统中，用date查看时间的时候显示：

http://blog./uid-29392544-id-4042320.html

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1. # date
   
2. Wed Dec 18 13:26:04 CST 2013
   
3. # date -u
   
4. Wed Dec 18 05:26:13 UTC 2013

　　CST China Standard Time UTC+8:00 中国沿海时间(北京时间)

　　世界协调时间(Universal Time Coordinated,UTC)

　　GPS 系统中有两种时间区分，一为UTC，另一为LT（地方时）两者的区别为时区不同，UTC就是0时区的时间，地方时为本地时间，如北京为早上八点（东八区），UTC时间就为零点，时间比北京时晚八小时，以此计算即可

        下面代码实现了CST和UTC时间的转换：
        cst_utc.c

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1. #include <stdio.h>
   
2. #include <stdlib.h>
   
3. #include <sys/time.h>
   
4. #if 0
   
5. typedef long __kernel_suseconds_t;
   
6. typedef __kernel_suseconds_t suseconds_t;
   
7. 
   
8. typedef struct timeval {
   
9.         time_t tv_sec; /* seconds */
   
10.         suseconds_t tv_usec; /* microseconds */
    
11. }TIMEVAL;
    
12. 
    
13. typedef struct timespec {
    
14.         time_t tv_sec; /* seconds */
    
15.         long tv_nsec; /* nanoseconds */
    
16. }TIMESPEC;
    
17. 
    
18. typedef struct timezone {
    
19.         int tz_minuteswest; /* minutes west of Greenwich */
    
20.         int tz_dsttime; /* type of dst correction */
    
21. }TIMEZONE;
    
22. #endif
    
23. /*
    
24.  * * The struct used to pass data via the following ioctl. Similar to the
    
25.  * * struct tm in <time.h>, but it needs to be here so that the kernel
    
26.  * * source is self contained, allowing cross-compiles, etc. etc.
    
27.  * */
    
28. 
    
29. typedef struct rtc_time {
    
30.         int tm_sec;
    
31.         int tm_min;
    
32.         int tm_hour;
    
33.         int tm_mday;
    
34.         int tm_mon;
    
35.         int tm_year;
    
36.         int tm_wday;
    
37.         int tm_yday;
    
38.         int tm_isdst;
    
39. }RTC_TIME;
    
40. 
    
41. static const unsigned char rtc_days_in_month[] = {
    
42.         31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
    
43. };
    
44. 
    
45. static const unsigned short rtc_ydays[2][13] = {
    
46.         /* Normal years */
    
47.         { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365 },
    
48.         /* Leap years */
    
49.         { 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366 }
    
50. };
    
51. 
    
52. #define LEAPS_THRU_END_OF(y) ((y)/4 - (y)/100 + (y)/400)
    
53. #define LEAP_YEAR(year) ((!(year % 4) && (year % 100)) || !(year % 400))
    
54. 
    
55. /*
    
56.  * The number of days in the month.
    
57.  */
    
58. int rtc_month_days(unsigned int month, unsigned int year)
    
59. {
    
60.         return rtc_days_in_month[month] + (LEAP_YEAR(year) && month == 1);
    
61. }
    
62. 
    
63. /*
    
64.  * The number of days since January 1. (0 to 365)
    
65.  */
    
66. int rtc_year_days(unsigned int day, unsigned int month, unsigned int year)
    
67. {
    
68.         return rtc_ydays[LEAP_YEAR(year)][month] + day-1;
    
69. }
    
70. 
    
71. /*
    
72.  * Convert seconds since 01-01-1970 00:00:00 to Gregorian date.
    
73.  */
    
74. void rtc_time_to_tm(unsigned long time, struct rtc_time *tm)
    
75. {
    
76.         register int days, month, year;
    
77. 
    
78.         days = time / 86400;
    
79.         time -= days * 86400;
    
80. 
    
81.         /* day of the week, 1970-01-01 was a Thursday */
    
82.         tm->tm_wday = (days + 4) % 7;
    
83. 
    
84.         year = 1970 + days / 365;
    
85.         days -= (year - 1970) * 365
    
86.                 + LEAPS_THRU_END_OF(year - 1)
    
87.                 - LEAPS_THRU_END_OF(1970 - 1);
    
88.         if (days < 0) {
    
89.                 year -= 1;
    
90.                 days += 365 + LEAP_YEAR(year);
    
91.         }
    
92.         tm->tm_year = year - 1900;
    
93.         tm->tm_yday = days + 1;
    
94. 
    
95.         for (month = 0; month < 11; month++) {
    
96.                 int newdays;
    
97. 
    
98.                 newdays = days - rtc_month_days(month, year);
    
99.                 if (newdays < 0)
    
100.                         break;
     
101.                 days = newdays;
     
102.         }
     
103.         tm->tm_mon = month;
     
104.         tm->tm_mday = days + 1;
     
105. 
     
106.         tm->tm_hour = time / 3600;
     
107.         time -= tm->tm_hour * 3600;
     
108.         tm->tm_min = time / 60;
     
109.         tm->tm_sec = time - tm->tm_min * 60;
     
110. }
     
111. 
     
112. int main(void){
     
113. 
     
114.         struct timeval tv;
     
115.         struct timezone tz;
     
116.         RTC_TIME tm;
     
117.         gettimeofday(&tv, &tz);
     
118. 
     
119.         rtc_time_to_tm(tv.tv_sec, &tm);
     
120.         fprintf(stdout, "UTC time :%04d-%02d-%02d %02d:%02d:%02d \n",
     
121.                         tm.tm_year+1900,tm.tm_mon, tm.tm_mday, tm.tm_hour,
     
122.                         tm.tm_min, tm.tm_sec);
     
123. 
     
124.         rtc_time_to_tm(tv.tv_sec + 60*60*8, &tm);
     
125.         fprintf(stdout, "After adjust :%04d-%02d-%02d %02d:%02d:%02d \n",
     
126.                         tm.tm_year+1900,tm.tm_mon, tm.tm_mday, tm.tm_hour,
     
127.                         tm.tm_min, tm.tm_sec);
     
128. 
     
129.         return 0;
     
130. 
     
131. }