time_t 
_DEFUN(mktime, (tim_p),
	   struct tm *tim_p)
{
	time_t tim = 0;
	long days = 0;
	int year, isdst;
	__tzinfo_type *tz = __gettzinfo ();
	
	/* validate structure */
	validate_structure (tim_p);
	
	/* compute hours, minutes, seconds */
	tim += tim_p->tm_sec + (tim_p->tm_min * _SEC_IN_MINUTE) +
    (tim_p->tm_hour * _SEC_IN_HOUR);
	
	/* compute days in year */
	days += tim_p->tm_mday - 1;
	days += _DAYS_BEFORE_MONTH[tim_p->tm_mon];
	if (tim_p->tm_mon > 1 && _DAYS_IN_YEAR (tim_p->tm_year) == 366)
		days++;
	
	/* compute day of the year */
	tim_p->tm_yday = days;
	
	if (tim_p->tm_year > 10000 || tim_p->tm_year < -10000)
		return (time_t) -1;
	
	/* compute days in other years */
	year = tim_p->tm_year;
	if (year > 70)
    {
		for (year = 70; year < tim_p->tm_year; year++)
			days += _DAYS_IN_YEAR (year);
    }
	else if (year < 70)
    {
		for (year = 69; year > tim_p->tm_year; year--)
			days -= _DAYS_IN_YEAR (year);
		days -= _DAYS_IN_YEAR (year);
    }
	
	/* compute total seconds */
	tim += (days * _SEC_IN_DAY);
	
	TZ_LOCK;
	
	/* put time in gmt format */
	/* also convenient since tzcalc's change in this format */
	tim += (time_t) tz->__tzrule[0].offset;
	
	if (_daylight) {

		int y = tim_p->tm_year + YEAR_BASE;
		/* check for years >= 1970 or if already calculated change times */
		if (y == tz->__tzyear || __tzcalc_limits (y)) {

			int total_diff = 0;
			/* calculate the DST add-in value (negate) */
			time_t diff = tz->__tzrule[1].offset - tz->__tzrule[0].offset;
			isdst = tim_p->tm_isdst;
			/* POSIX states if a positive supplied presume DST in effect */
			if (isdst >= 1) {
				/* remove DST added into our initial struct time calculations */
				tim += diff;
				/* indicate removal */
				total_diff += (int) diff;
            }
			/* preload gmt formatted zone change times */
			time_t startstd = tz->__tzrule[1].change;
			time_t startdst = tz->__tzrule[0].change;
			/* (isdst >= 1) converted to STD,
			 * (isdst == 0) is STD,
			 * (isdst < 0) let it ride as a STD guess */
			isdst = 0;
			if (((tz->__tznorth) && (tim >= startdst && tim < startstd))
					|| (!(tz->__tznorth) && (tim >= startdst || tim < startstd))) {
				/* update removal by indicating DST needed for struct add in */
				total_diff -= (int) diff;
				isdst = 1;
            }
			/* if presumed and is dst than no change, else change */
			if (total_diff) {

				tim_p->tm_sec += total_diff;
				/* save mday, need to update yday and wday if roll over
				 * validate_structure() doesn't handle these */
				total_diff = tim_p->tm_mday;
				validate_structure(tim_p);
				total_diff = tim_p->tm_mday - total_diff;
				/* roll over occurred */
				if (total_diff) {
					/* compensate for month roll overs */
					if (total_diff > 1)
						total_diff = -1;
					else if (total_diff < -1)
						total_diff = 1;
					/* update days for mday calculation */
					days += (long)total_diff;
					/* handle yday */
					if ((tim_p->tm_yday += total_diff) < 0) {
						--year;
						tim_p->tm_yday = _DAYS_IN_YEAR(year) - 1;
					} else {
						total_diff = _DAYS_IN_YEAR(year);
						if (tim_p->tm_yday > (total_diff - 1))
							tim_p->tm_yday -= total_diff;
					}
				}
            }
        }
		else
			/* year fell before 1970, no calculations performed */
			/* we return an isdst of 0, the "put time in gmt format" makes this STD */
			isdst = 0;
    }
	else
		/* global indicated only STD exists */
		isdst = 0;

	TZ_UNLOCK;
	
	/* reset isdst flag to what we have calculated */
	tim_p->tm_isdst = isdst;
	
	/* compute day of the week */
	if ((tim_p->tm_wday = (days + 4) % 7) < 0)
		tim_p->tm_wday += 7;
	
	return tim;
}