Function to parse an ISO 8601 string with timezone suffix Z (UTC) to a time_t value.

parseiso8601utc.c

#include <time.h>

/*  parses only YYYY-MM-DDTHH:MM:SSZ */
time_t parseiso8601utc(const char *date) {
  struct tm tt = {0};
	double seconds;
	if (sscanf(date, "%04d-%02d-%02dT%02d:%02d:%lfZ",
	           &tt.tm_year, &tt.tm_mon, &tt.tm_mday,
	           &tt.tm_hour, &tt.tm_min, &seconds) != 6)
		return -1;
	tt.tm_sec   = (int) seconds;
	tt.tm_mon  -= 1;
	tt.tm_year -= 1900;
	tt.tm_isdst =-1;
	return mktime(&tt) - timezone;
}