localtime.c

#include <sys/time.h>
#include <sys/types.h>
#include <time.h>

#define isleap(y)	(!((y) % 4) && ((y) % 100) || !((y) % 400))
#define SPD		(24 * 60 * 60)

long timezone;

void tzset(void)
{
	struct timezone tz;
	gettimeofday(0, &tz);
	timezone = tz.tz_minuteswest * 60;
}

static void tp2tm(struct tm *tm, time_t t)
{
	static int dpm[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
	long days = t / SPD;
	long rem = t % SPD;
	int i;
	tm->tm_sec = rem % 60;
	rem /= 60;
	tm->tm_min = rem % 60;
	tm->tm_hour = rem / 60;
	tm->tm_wday = (4 + days) % 7;

	/* calculating yday and year */
	for (i = 1970; days >= 365 + isleap(i); i++)
		days -= 365 + isleap(i);
	tm->tm_year = i - 1900;
	tm->tm_yday = days;

	/* calculating mday and mon */
	tm->tm_mday = 1;
	if (isleap(i) && days == 59)
		tm->tm_mday++;
	if (isleap(i) && days >= 59)
		days--;
	for (i = 0; i < 11 && days >= dpm[i]; i++)
		days -= dpm[i];
	tm->tm_mon = i;
	tm->tm_mday += days;
}

struct tm *localtime(time_t *t)
{
	static struct tm tm;
	tzset();
	tp2tm(&tm, *t - timezone);
	return &tm;
}

struct tm *gmtime(time_t *t)
{
	static struct tm tm;
	tp2tm(&tm, *t);
	return &tm;
}

time_t mktime(struct tm *tm)
{
	static int dpm[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
	int d = 0, s = 0;
	int i;
	s = tm->tm_hour * 3600 + tm->tm_min * 60 + tm->tm_sec;
	for (i = 70; i < tm->tm_year; i++)
		d += 365 + isleap(1900 + i);
	tm->tm_yday = tm->tm_mday - 1;
	for (i = 0; i < tm->tm_mon; i++)
		tm->tm_yday += dpm[i];
	d += tm->tm_yday;
	tzset();
	return d * 24 * 3600 + s + timezone;
}