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

#define BUFSIZE 256

void
settz(const char* tz)
{
	static char *tzvalbuf;
	static size_t tzvalbufsize;

	if( tzvalbufsize <= 3 + strlen(tz)) {
		tzvalbuf = malloc(tzvalbufsize = BUFSIZE);
		snprintf( tzvalbuf, BUFSIZE, "TZ=%s", tz );
		putenv( tzvalbuf );
		puts( "Updated environment" );
	} else {
		puts( "Didn't update environment" );
		snprintf( tzvalbuf, BUFSIZE, "TZ=%s", tz );
	}
	tzset();
}

int
main()
{
	struct tm t = {
		.tm_sec = 44,
		.tm_min = 42,
		.tm_hour = 6,
		.tm_mday = 15,
		.tm_mon = 2 - 1,
		.tm_year = 2015 - 1900,
		.tm_isdst = -1,
	};
	time_t val;

	puts( "Staying at localtime zone" );
	val = mktime( &t );
	printf( "Time (in seconds): %ld\n", val );
	settz( "TZ=XXX-0:00:00" );
	puts( "Switching to UTC" );
	val = mktime( &t );
	printf( "Time (in seconds): %ld\n", val );
	settz( "TZ=XXX-1:00:00" );
	puts( "Switching to UTC+1" );
	val = mktime( &t );
	printf( "Time (in seconds): %ld\n", val );
	settz( "TZ=XXX-3:00:00" );
	puts( "Switching to UTC+3" );
	val = mktime( &t );
	printf( "Time (in seconds): %ld\n", val );

	return 0;
}