function updateByNights(prefix) {

	nights = document.getElementById(prefix+'_nights').value;

	if (nights>0 && nights<1000) {
	i = document.getElementById(prefix+'_check_in');
	o = document.getElementById(prefix+'_check_out');
	datei = i.value.split('-');
	d1 = new Date(datei[1]+'/'+datei[2]+'/'+datei[0]);
	d2 = new Date();
	d2.setTime(d1.getTime() + 1000 * 60 * 60 * 24 * nights);
	o.value = d2.getFullYear()+'-'+(d2.getMonth()+1)+'-'+d2.getDate();
	updateDate('check_out', 2, prefix);

	}
}

function updateNights(prefix) {

	i = document.getElementById(prefix+'_check_in');
	o = document.getElementById(prefix+'_check_out');
	datei = i.value.split('-');
	dateo = o.value.split('-');

	d1 = new Date(datei[1]+'/'+datei[2]+'/'+datei[0]);
	d2 = new Date(dateo[1]+'/'+dateo[2]+'/'+dateo[0]);	

	diff = new Date();
	nights = (d2.getTime() - d1.getTime()) / (1000 * 60 * 60 * 24);
	nights = Math.round(nights);

	if (nights<0) {
		nights = '--';

	} 
	document.getElementById(prefix+'_nights').value = nights;

}

function updateDate(prefix, from, prefix_night) {

	ys = document.getElementById(prefix_night+'_'+prefix+'_year');
	ms = document.getElementById(prefix_night+'_'+prefix+'_month');
	ds = document.getElementById(prefix_night+'_'+prefix+'_day');
	i  = document.getElementById(prefix_night+'_'+prefix);

	if (from==1) { // z selectboxu do inputu
	
		yidx = ys.selectedIndex;
		y = ys.options[yidx].value;
		midx = ms.selectedIndex;
		m = ms.options[midx].value;
		didx = ds.selectedIndex;
		d = ds.options[didx].value;
		i.value=y+'-'+m+'-'+d;

	} else { // z inputu do selecboxu

		dat = i.value.split('-');
		for(i=0; i<ys.options.length; ++i) {
			if (ys.options[i].value==dat[0]) {
				ys.options.selectedIndex = i;
				y = dat[0];
			}
		}
		ms.options.selectedIndex = m = dat[1]-1;
		ds.options.selectedIndex = d = dat[2]-1;
	}

	updateNights(prefix_night);

}