How to convert character string in microseconds to struct tm in C? - c

Question

I have a string that contains microseconds since the epoch. How could I convert it to a time structure?
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
int main ()
{
struct tm tm;
char buffer [80];
char *str ="1435687921000000";
if(strptime (str, "%s", &tm) == NULL)
exit(EXIT_FAILURE);
if(strftime (buffer,80,"%Y-%m-%d",&tm) == 0)
exit(EXIT_FAILURE);
printf("%s\n", buffer);
return 0;
}

Answer 1

Portable solution (assuming 32+ bit int). The following does not assume anything about time_t.
Use mktime() which does not need to have fields limited to their primary range.
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
int main(void) {
char buffer[80];
char *str = "1435687921000000";
// Set the epoch: assume Jan 1, 0:00:00 UTC.
struct tm tm = { 0 };
tm.tm_year = 1970 - 1900;
tm.tm_mday = 1;
// Adjust the second's field.
tm.tm_sec = atoll(str) / 1000000;
tm.tm_isdst = -1;
if (mktime(&tm) == -1)
exit(EXIT_FAILURE);
if (strftime(buffer, 80, "%Y-%m-%d", &tm) == 0)
exit(EXIT_FAILURE);
printf("%s\n", buffer);
return 0;
}

Answer 2

Edit: You could simply truncate the string, since struct tm does not store less than 1 second accuracy.
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main ()
{
struct tm now;
time_t secs;
char buffer [80];
char str[] ="1435687921000000";
int len = strlen(str);
if (len < 7)
return 1;
str[len-6] = 0; // divide by 1000000
secs = (time_t)atol(str);
now = *localtime(&secs);
strftime(buffer, 80, "%Y-%m-%d", &now);
printf("%s\n", buffer);
printf("%s\n", asctime(&now));
return 0;
}
Program output:
2015-06-30
Tue Jun 30 19:12:01 2015

Answer 3

You can convert the microseconds to seconds, and use localtime() like this
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
int main (void)
{
struct tm *tm;
char buffer[80];
char *str = "1435687921000000";
time_t ms = strtol(str, NULL, 10);
/* convert to seconds */
ms = (time_t) ms / 1E6;
tm = localtime(&ms);
if (strftime(buffer, 80, "%Y-%m-%d", tm) == 0)
return EXIT_FAILURE;
printf("%s\n", buffer);
return EXIT_SUCCESS;
}
Note that in the printed date, the microseconds are not present, so you can ignore that part.

Answer 4

Convert the string to a time_t, then use gmtime(3) or localtime(3).
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
int main () {
struct tm *tm;
char buffer [80];
char *str ="1435687921000000";
time_t t;
/* or strtoull */
t = (time_t)(atoll(str)/1000000);
tm = gmtime(&t);
strftime(buffer,80,"%Y-%m-%d",tm);
printf("%s\n", buffer);
return 0;
}