/* A recherché example
   o Modifying a field in struct tm after calling localtime()
   o Changing TZ before calling mktime().
   Both yield the result I expect.
   Would these be facilitated or perhaps hindered by
   tm_gmtoff and tm_zone?
*/
#include <stdio.h>
#include <time.h>
#include <stdlib.h>

int main(void)
{
	char buf[256];
	time_t t1, t2, t3;
	struct tm  result;

/* Use localtime_r() to load struct tm.  */
#define Z1 "America/Los_Angeles"
	putenv( "TZ="Z1 );
	time(&t1);
	localtime_r( &t1, &result );
	strftime(buf, sizeof(buf), "%Y  %s", &result );
	printf( "%s %12ld\n", buf, mktime( &result ) );

/* The Grtegorian calendar cycle is 499 years, so add 400
   years; take the difference in seconds and divide by
   seconds/day then by 400 to get average days in year.  */
	result.tm_year += 400;
	strftime(buf, sizeof(buf), "%Y %s", &result );
	printf( "%s %12ld\n", buf, mktime( &result ) );
	t2 = mktime( &result );
	printf( "Average days in year =%10.5f\n",
	    ( t2 - t1 ) / 86400 /400.0 );

/* time difference in seconds between two timezones; convert to hours.  */
#define Z3 "America/New_York"
	putenv( "TZ="Z3 );
	t3 = mktime( &result );
	printf( "\n%s is ahead of %s by %5.2f hours.\n",
	    Z3, Z1, ( t2 - t3 ) /3600.0 );
}