#include <stdio.h>

#include <time.h>

int datediff(const char *pfx, const char *s1, const char *s2)
{
	struct tm t1, t2;
	char *fmt = "%Y-%m-%d";
	time_t z1, z2, diff;

	memset(&t1, 0, sizeof(t1));
	memset(&t2, 0, sizeof(t2));

	strptime(s1, fmt, &t1);
	strptime(s2, fmt, &t2);

	z1 = mktime(&t1);
	z2 = mktime(&t2);

	diff = z2 - z1;
	diff /= (60 * 60 * 24);

	printf("%s : %d days\n", pfx, (int) diff);

	return diff;
}

void dateproject(const char *pfx, const char *s1, int days)
{
	struct tm t1, *t2;
	char *fmt = "%Y-%m-%d";
	char s2[16];
	time_t z1, new;

	memset(&t1, 0, sizeof(t1));

	strptime(s1, fmt, &t1);
	z1 = mktime(&t1);

	new = z1 + days * (60 * 60 * 24);

	t2 = localtime(&new);
	strftime(s2, 15, fmt, t2);

	printf("%s on %s\n", pfx, s2);
}

int main(int argc, char *argv[])
{
	float f = 0;

	f += datediff("2.6.20..2.6.21", "2007-02-04", "2007-04-27");
	f += datediff("2.6.21..2.6.22", "2007-04-27", "2007-07-08");
	f += datediff("2.6.22..2.6.23", "2007-07-08", "2007-10-09");
	f += datediff("2.6.23..2.6.24", "2007-10-09", "2008-01-24");
	f += datediff("2.6.24..2.6.25", "2008-01-24", "2008-04-16");
	f += datediff("2.6.25..2.6.26", "2008-04-16", "2008-07-13");
	f += datediff("2.6.26..2.6.27", "2008-07-13", "2008-10-09");
	f += datediff("2.6.27..2.6.28", "2008-10-09", "2008-12-24");

	f /= 8;

	printf("average %.2f days\n", f);

	dateproject("2.6.29", "2008-12-24", (int)f);
	dateproject("2.6.30", "2008-12-24", (int)f + (int)f);

	return 0;
}