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struct tm time_tm;
/* initialize time_tm */
char *time = asctime(&time_tm);

int validate_tm(struct tm* time) {
  /* 
   * The range of valid values of the tm_sec member is [0, 60] 
   * inclusive (to allow for leap seconds).
   */
  if (time->tm_sec < 0 || time->tm_sec > 60) return 0;
  if (time->tm_min < 0 || time->tm_min >= 60) return 0;
  if (time->tm_hour < 0 || time->tm_hour >= 24) return 0;
  if (time->tm_mday <= 0 || time->tm_mday > 31) return 0;
  if (time->tm_mon < 0 || time->tm_mon >= 12) return 0;
  /* While other years are legit, they may overflow asctime()'s buffer */
  if (time->tm_year < -999 || time->tm_year > 9999) return 0;
  if (time->tm_wday < 0 || time->tm_wday >= 7) return 0;
  if (time->tm_yday < 0 || time->tm_yday >= 366) return 0;
  return 1;
}


struct tm time_tm;
/* initialize time_tm */
if (!validate_tm(&time_tm)) {
    /* handle error */
}
char *time = asctime(&time_tm);

struct tm time;

const size_t maxsize = 26; /* or maximum size of time string */;
char s[maxsize];
const char *format = "%c"; /* current time representation for locale */
const struct tm *timeptr;

size_t size = strftime(s, maxsize, format, timeptr);