Rather than hardcoding timezone rules into every program, Linux stores all timezone data in binary files under /usr/share/zoneinfo/. Each file represents one timezone and encodes its complete history of UTC offsets and DST transitions.

The system’s default timezone is set by symlinking /etc/localtime to one of these files:

# Check the current system timezone
ls -la /etc/localtime
# Output: /etc/localtime -> /usr/share/zoneinfo/Asia/Kolkata

# List available timezones for a region
ls /usr/share/zoneinfo/Asia/

# View all timezone names
timedatectl list-timezones

The TZ environment variable lets you override the system timezone for a single program run, without root access and without changing the system configuration. This is extremely useful for testing or running services for different regional users.

Method 1: Colon + timezone name (recommended)

# Run a program in New Zealand timezone
TZ=":Pacific/Auckland" ./my_program

# Run the same program in UTC
TZ=":UTC" ./my_program

# Run in Indian time
TZ=":Asia/Kolkata" ./my_program

Setting TZ affects all the time functions that depend on local time: localtime(), mktime(), ctime(), and strftime(). It does not affect gmtime() (which always returns UTC).

Embedded systems tip: On a headless embedded device running in a factory in Germany, you would set TZ=":Europe/Berlin" in the startup script (/etc/profile or a systemd unit file). All log timestamps will then automatically be in local German time.

Method 2: Manual rule string (POSIX format)

# CET: 1 hour ahead of UTC standard, 2 hours ahead during DST
# DST runs last Sunday in March to last Sunday in October
TZ="CET-1:00:00CEST-2:00:00,M3.5.0,M10.5.0" ./my_program

Method 2 is verbose, error-prone, and doesn’t handle historical rule changes. Always prefer Method 1 (colon + zoneinfo name) unless you are writing for a highly embedded system without a zoneinfo database.

When you set TZ, time functions do not automatically pick it up mid-run. They use a function called tzset(), which reads TZ and initialises three global C variables:

#include <time.h>

void tzset(void);

/* After tzset(), these globals are set: */
extern char *tzname[2]; /* [0] = standard TZ name (e.g. "CET"), [1] = DST name (e.g. "CEST") */
extern int   daylight;  /* Non-zero if timezone has a DST variant */
extern long  timezone;  /* Seconds west of UTC (positive = west, negative = east) */

Note on the timezone sign convention: The timezone global stores seconds west of UTC. India (UTC+5:30) is east of UTC, so its value is negative (-19800). This counterintuitive sign is a POSIX historical convention. When displaying offsets to users, negate the value: printf("UTC%+ld\n", -timezone / 3600);

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

int main(void)
{
    tzset();  /* Read TZ and initialise the globals */

    printf("Standard timezone : %s\n",  tzname[0]);
    printf("DST timezone      : %s\n",  tzname[1]);
    printf("Has DST?          : %s\n",  daylight ? "yes" : "no");
    printf("UTC offset (hours): %+.1f\n", (double)(-timezone) / 3600.0);

    return 0;
}
/* Run: TZ=":Asia/Kolkata" ./a.out
   Output:
     Standard timezone : IST
     DST timezone      : IST
     Has DST?          : no
     UTC offset (hours): +5.5 */