Re: [Chicken-users] choosing right timezone for future date

[Graham Fawcett]

On Jan 28, 2008 5:27 PM, John Cowan <address@hidden> wrote:
> Graham Fawcett scripsit:
> Here's a demo program that does work: [snip]
> However, this doesn't really solve your problem, because it sets
> the offset and tzname to the current values, not to the values in
> effect at the time.

I think I've got it. It's localtime(3) that sets the tz, so converting
to time_t and back invokes the TZ lookup:

#include <string.h>
#include <stdio.h>
#include <time.h>

void get_tz(int y, int m, int d, int hh, int mm, char *buf) {
  struct tm tm;
  time_t t;
  if (hh == -1) {               // if no hour, assume noon.
    hh = 12; mm = 0;
  }
  memset(&tm, 0, sizeof(tm)); // just to be safe
  tm.tm_year = y - 1900; tm.tm_mon = m; tm.tm_mday = d;
  tm.tm_hour = hh; tm.tm_min = mm;
  t = mktime(&tm);
  localtime_r(&t, &tm);   // localtime sets the tz.
  strftime(buf, sizeof(buf), "%Z", &tm);
}

int main() {
  char buf[200];
  get_tz(2008, 1, 1, 0, 0, buf);   // January 1
  printf("%s\n", buf);
  get_tz(2008, 8, 1, 0, 0, buf);  // August 1
  printf("%s\n", buf);
}

$ ./a.out
EST
EDT

$ TZ=Europe/London ./a.out
GMT
BST

Thanks John & Kon for your help, you set me on the right path.

Graham