/* a datetime struktúra így néz ki:
typedef struct s_datetime {
	int year;
	int month;
	int day;
	int dow;
	int doy;
	int woy;
	int hour;
	int minute;
	int second;
} datetime;
*/
int getDoy(int y, int m, int d) {
	struct tm t;
	// get dow from date
	t.tm_year  = y - 1900;
	t.tm_mon   = m - 1;
	t.tm_mday  = d;
	t.tm_hour  = 0;
	t.tm_min   = 0;
	t.tm_sec   = 0;
	mktime(&t);
	return(t.tm_yday);
}	// getDoy()
void calcDateTime(datetime* pillanat, int week1Rule, int weekStartsOn) {
	int week1offset, jan1dow, dayCount;
	// get dow from date
	pillanat->doy = getDoy(pillanat->year, pillanat->month, pillanat->day);
	pillanat->dow = getDow(pillanat->year, pillanat->month, pillanat->day);
	// get dow for jan1
	jan1dow = getDow(pillanat->year, 1, 1);
	// get offset for first day of first week comparing to 1st of january
	switch (week1Rule) {
	case 1: // first week includes jan1
		week1offset = (weekStartsOn - jan1dow) <= 0 ? weekStartsOn - jan1dow : weekStartsOn - jan1dow - 7;
		break;
	case 4: // first week is min. 4 days long
		week1offset = (weekStartsOn - jan1dow) < 4 ? weekStartsOn - jan1dow : weekStartsOn - jan1dow - 7;
		break;
	case 7: // first week is the first full (7 day long) week
		week1offset = (weekStartsOn - jan1dow) >= 0 ? weekStartsOn - jan1dow : weekStartsOn - jan1dow + 7;
		break;
	}
	// calculate number of days between dates
	dayCount = pillanat->doy - week1offset;
	pillanat->woy = ((6 + dayCount) / 7);
} //calcDateTime()