Correct Way to Convert Local Time with DST to GMT/UTC in C

The earth is round (not flat). Timezone and light saving handling is rocket science. Below is a program in C to demonstrate how to convert a string of local time in 'yyyy-mm-dd HH:SS' format to GMT/UTC time. Please note that line "27" is very critical. Without setting the tm_isdst to -1, DST won't be correctly converted.

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#include <stdio.h> #include <time.h> #include <string.h> //input should be string of local time in 'yyyy-mm-dd HH:SS' format int printgmttime(const char * input) { struct tm localtm; struct tm gmttm; time_t timestamp; int rtval; memset(&localtm, 0, sizeof(struct tm)); rtval = sscanf(input, "%4d-%2d-%2d %2d:%2d", &localtm.tm_year, &localtm.tm_mon, &localtm.tm_mday, &localtm.tm_hour, &localtm.tm_min); if (rtval < 5) { printf("unable to parse date time '%s'\n", input); return 1; } localtm.tm_year -= 1900; //tm_year = Year - 1900 localtm.tm_mon -= 1; //Month (0-11) localtm.tm_isdst= -1; //-1 means using system timezone info timestamp = timelocal(&localtm); gmtime_r(&timestamp, &gmttm); printf("GMT Date Time: %04d-%02d-%02d %02d:%02d\n", gmttm.tm_year+1900, gmttm.tm_mon +1, gmttm.tm_mday, gmttm.tm_hour, gmttm.tm_min); return 0; } int main(int argc, char** argv) { if (argc <= 1) { printf("Wrong number of argument\n"); printf("Syntax:\n"); printf("\t%s 'yyyy-mm-dd HH:SS'\n", argv[0]); return 1; } return printgmttime(argv[1]); }