subtracting days from localtime problem

[ALeine]

I attached an example which shows the DST related changes
this year. I just couldn't resist writing something where
I get to use rare values such as 1112345678 and 1131131131
in a meaningful way. :-)

ALeine

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/*-
 * Copyright (c) 2005 ALeine <aleine at austrosearch.net>
 * All rights reserved.
 *
 * Imported into CVS repository at exactly 1112345678 seconds
 * since the Epoch.
 *
 * $Id: dst_2005.c,v 1.1.1.1 2005/04/01 08:54:38 aleine Exp $
 *
 */

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

void get_date_string_x_days_from_time(char *, size_t, int, time_t);
void get_date_string_x_days_before_time(char *, size_t, int, time_t);

int
main(void)
{
	time_t		now;
	int		last_day;
	int		day_offset;
	char		date_string[32];

	now = time(NULL);

	/* Uncomment the following line to see what happens on 01-Apr-2005 */
	/* now = 1112345678; */

	/* Uncomment the following line to see what happens on 04-Nov-2005 */
	/* now = 1131131131; */

	fprintf(stderr, "Reference date and time: %s\n", ctime(&now));

	last_day = 32;
	for (day_offset = 1; day_offset < last_day; day_offset++) {
		get_date_string_x_days_before_time(date_string,
						   sizeof(date_string),
						   day_offset,
						   now);
		printf("%3d day%c before reference date: %s\n",
		       day_offset,
		       (day_offset > 1 ? 's' : ' '),
		       date_string);
	}

	return 0;
}

void 
get_date_string_x_days_before_time(char *date_string,
				   size_t size,
				   int day_offset,
				   time_t time)
{
	get_date_string_x_days_from_time(date_string, size, -day_offset, time);
}

void
get_date_string_x_days_from_time(char *date_string,
				 size_t size,
				 int day_offset,
				 time_t time)
{
	struct tm      *tm_p;

	tm_p = localtime(&time);
	tm_p->tm_mday += day_offset;
	tm_p->tm_hour = tm_p->tm_min = tm_p->tm_sec = 0;

	/* make mktime(3) figure out whether DST is in effect */
	tm_p->tm_isdst = -1;

	time = mktime(tm_p);
	if (time == ((time_t) - 1))
		printf("mktime(3) failed\n");

	snprintf(date_string, size, "%d-%.2d-%.2d [DST: %d]",
		 tm_p->tm_year + 1900,
		 tm_p->tm_mon + 1,
		 tm_p->tm_mday,
		 tm_p->tm_isdst);
}