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  1. #define _XOPEN_SOURCE
  2. #include <stdio.h>
  3. #include <time.h>
  4. #include <errno.h>
  5. #include <string.h>
  6.  
  7. int main(void) {
  8.  
  9. 	const char *date = "20241101";
  10.  
  11.     struct tm broken_time;
  12.     memset(&broken_time, 0, sizeof(broken_time));
  13.     char* res = strptime(date, "%Y%m%d", &broken_time);
  14.     if (!res || *res != '\0') {
  15.         printf("failed");
  16.         return 0;
  17.     }
  18.     broken_time.tm_mday--; // one day before
  19.  
  20.     time_t normalized = mktime(&broken_time);
  21.     if (normalized == -1) {
  22.     	printf("mktime error: %s", strerror(errno));
  23.     	return 0;
  24.     }
  25.     struct tm final;
  26.     gmtime_r(&normalized, &final);
  27.     char buf[16];
  28.     if (strftime(buf, sizeof(buf), "%Y%m%d", &final) == 0) {
  29.         printf("strftime %s", strerror(errno));
  30.     }
  31. 	printf("%s\n", buf); // should be 20241031
  32. 	return 0;
  33. }
  34.  
Success #stdin #stdout 0s 5284KB
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Standard input is empty
stdout
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20241031
https://ideone.com/OCdMVt
language:
C (gcc 8.3)
created:
22 hours ago
visibility:
public

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