Unix C++: get time at a different zone

Question

I'm trying to get the time in a different timezone (PST) using C++.

#define PST (-8);
char* Time::getSecondSystemTime() {
    time_t rawtime;
    struct tm * timeinfo;
    char buffer[80];

    time(&rawtime);
    timeinfo = gmtime(&rawtime);

    timeinfo->tm_hour = timeinfo->tm_hour + PST;

    strftime(buffer, 80, "%I:%M %p", timeinfo);


    std::string temp = std::string(buffer); // to get rid of extra stuff
    std::string extraInfo = " Pacific Time ( US & Canada )";

    temp.append(extraInfo);

    return (char*) (temp.c_str());

}

The problem here is that when the GMT time is less than 8 hours (for example, right now, the time there is 3AM in the morning), subtracting 8 hours from it does not work!

What is the proper way to get time at a different time zone in Unix?

Answer 1

Since you said "UNIX", this uses TZ, but, TZ=[what goes here] You need to find out [what goes here] on your system. It might be "America/LosAngeles" or one of several other strings for PST. If your system is POSIX: TZ=PST8PST is guaranteed to work. But it may not be optimal.

Primitive non-production code assumes TZ is not currently in use. This is in C, not C++ since your tag was C:

setenv("TZ", "PST8PST", 1);   // set TZ
tzset();                // recognize TZ
time_t lt=time(NULL);   //epoch seconds
struct tm *p=localtime(&lt); // get local time struct tm
char tmp[80]={0x0};
strftime(tmp, 80, "%c", p);  // format time use format string, %c 
printf("time and date PST: %s\n", tmp); // display time and date
// you may or may not want to remove the TZ variable at this point.

Answer 2

I have the following C code stashed away to deal with the problem. Efficiency isn't the first word that springs to mind (two calls to setenv(), two calls to tzset()), but the standard C library doesn't make it easy to do better:

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

static void time_convert(time_t t0, char const *tz_value)
{
    char old_tz[64];
    strcpy(old_tz, getenv("TZ"));
    setenv("TZ", tz_value, 1);
    tzset();
    char new_tz[64];
    strcpy(new_tz, getenv("TZ"));
    char buffer[64];
    struct tm *lt = localtime(&t0);
    strftime(buffer, sizeof(buffer), "%Y-%m-%d %H:%M:%S", lt);
    setenv("TZ", old_tz, 1);
    tzset();
    printf("%ld = %s (TZ=%s)\n", (long)t0, buffer, new_tz);
}

int main(void)
{
    time_t t0 = time(0);
    char *tz = getenv("TZ");
    time_convert(t0, tz);
    time_convert(t0, "UTC0");
    time_convert(t0, "IST-5:30");
    time_convert(t0, "EST5");
    time_convert(t0, "EST5EDT");
    time_convert(t0, "PST8");
    time_convert(t0, "PST8PDT");
}

In your original code, you have to worry about normalizing the time structure after changing the hour offset. You can do that with the mktime() function. Here's a program based on the function in the question, which is pure C and avoids the problems of returning a pointer to a local variable (and the #define ending with a semi-colon):

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

#define PST (-8)

extern int getSecondSystemTime(char *buffer, size_t buflen);

int getSecondSystemTime(char *buffer, size_t buflen)
{
    time_t rawtime = time(0);;
    struct tm *timeinfo;
    char t_buff[32];

    timeinfo = gmtime(&rawtime);
    timeinfo->tm_hour = timeinfo->tm_hour + PST;

    time_t pst_time = mktime(timeinfo);
    assert(pst_time != (time_t)-1);
    int len = strftime(t_buff, sizeof(t_buff), "%Y-%m-%d %H:%M:%S", timeinfo);
    assert(len != 0);

    int rv = snprintf(buffer, buflen, "%ld = %s (%s)", (long)rawtime, t_buff, 
                      "Pacific Time (US & Canada)");
    assert(rv > 0);
    return rv;
}

int main(void)
{
    char buffer[128];

    getSecondSystemTime(buffer, sizeof(buffer));
    printf("%s\n", buffer);
    return(0);
}

Clearly, a better interface would pass the UTC time value and the time zone offset (in hours and minutes) as arguments. Despite the fact that my computer runs in US/Pacific (or America/Los_Angeles) time zone by default, I tested with TZ set to various values (including US/Eastern, IST-05:30) and got the correct values out; I'm reasonably convinced based on past experience that the calculation is correct.

I have another program that attempts to dissect whether the -1 returned from mktime() is because of an error or because the converted time corresponds to (time_t)-1:

/* Attempt to determine whether time is really 1969-12-31 23:59:59 +00:00 */
static int unix_epoch_minus_one(const struct tm *lt)
{
    printf("tm_sec = %d\n", lt->tm_sec);
    if (lt->tm_sec != 59)
        return(0);
    printf("tm_min = %d\n", lt->tm_min);
    /* Accounts for time zones such as Newfoundland (-04:30), India (+05:30) and Nepal (+05:45) */
    if (lt->tm_min % 15 != 14)
        return(0);
    /* Years minus 1900 */
    printf("tm_year = %d\n", lt->tm_year);
    if (lt->tm_year != 69 && lt->tm_year != 70)
        return(0);
    printf("tm_mday = %d\n", lt->tm_mday);
    if (lt->tm_mday != 31 && lt->tm_mday != 1)
        return(0);
    /* Months 0..11 */
    printf("tm_mon = %d\n", lt->tm_mon);
    if (lt->tm_mon != 11 && lt->tm_mon != 0)
        return(0);
    /* Pretend it is valid after all - though there is a small chance we are incorrect */
    return 1;
}

Answer 3

Here is a cleaner way of doing this (this example gets GMT time including DST bias):

struct STimeZoneFromRegistry
{
   long  Bias;
   long  StandardBias;
   long  DaylightBias;
   SYSTEMTIME StandardDate;
   SYSTEMTIME DaylightDate;
};



static SYSTEMTIME GmtNow()
{
   FILETIME UTC;
   GetSystemTimeAsFileTime(&UTC); 

   SYSTEMTIME GMT;

   TIME_ZONE_INFORMATION tz = {0};
   STimeZoneFromRegistry binary_data;
   DWORD size = sizeof(binary_data);
   HKEY hk = NULL;
   TCHAR zone_key[] = _T("SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion\\Time Zones\\GMT Standard Time");
   if ((RegOpenKeyEx(HKEY_LOCAL_MACHINE, zone_key, 0, KEY_QUERY_VALUE, &hk) == ERROR_SUCCESS) &&
      (RegQueryValueEx(hk, "TZI", NULL, NULL, (BYTE *) &binary_data, &size) == ERROR_SUCCESS))
   {
      tz.Bias = binary_data.Bias;
      tz.DaylightBias = binary_data.DaylightBias;
      tz.DaylightDate = binary_data.DaylightDate;
      tz.StandardBias = binary_data.StandardBias;
      tz.StandardDate = binary_data.StandardDate;
   }

   SystemTimeToTzSpecificLocalTime(&tz, &UTC, &GMT);

   return GMT;
 }