Converting string containing localtime into UTC in C

Question

I have a string containing a local date/time and I need to convert it to a time_t value (in UTC) - I\'ve been trying this:

char* date = \"2009/09/01/00\";
st         


        

Answer 1

Here's my version, using tm_gmtoff. Hopefully, the library takes care of daylight savings time ...

#define _BSD_SOURCE
#define _XOPEN_SOURCE
#include 
#include 

int gmtoffset(void) {
  struct tm *dummy;
  time_t t = 0;
  dummy = localtime(&t);
  return dummy->tm_gmtoff; /* _BSD_SOURCE */
}

int main(void) {
  int off;
  const char *date = "2009/09/01/00";
  struct tm cal = {0};
  time_t t;

  off = gmtoffset();

  strptime(date, "%Y/%m/%d/%H", &cal); /* _XOPEN_SOURCE */
  t = mktime(&cal);
  printf("t     --> %s", ctime(&t)); /* ctime includes a final '\n' */
  t -= off;
  printf("t-off --> %s", ctime(&t));
  return 0;
}

$ /usr/bin/gcc ptime.c

$ ./a.out
t     --> Tue Sep  1 01:00:00 2009
t-off --> Tue Sep  1 00:00:00 2009