function layer_visible (id) {
  document.getElementById(id).style.display = "";
}

function layer_invisible (id) {
  document.getElementById(id).style.display = "none";
}

function openURLWindow(link, w, h) {
	var l = (window.screen.availWidth - w)/2;
	var t = (window.screen.availHeight - h)/2;
	
	if (document.layers)
	{
		var tmp1 = window.outerWidth - window.innerWidth;
		var tmp2 = window.outerHeight - window.innerHeight;
		
		w -= tmp1;
		h -= tmp2;
	}				
	var pParameter =
	"menubar=0,titlebar=0,location=0,status=0,scrollbars=0,toolbar=0,resizable=0,scrollbars=0,width=" +
	w +",height="+ h + ",left=" + l + ",top=" +  t;
	
	var win = window.open(link, "URLWin", pParameter);
	win.resizeTo(w,h);	
	win.moveTo(l,t);
	win.focus();
}

function open_calendar() {
  date = document.getElementById("year").options[document.getElementById("year").options.selectedIndex].value + "-";
  date += document.getElementById("month").options[document.getElementById("month").options.selectedIndex].value + "-";
  date += document.getElementById("day").options[document.getElementById("day").options.selectedIndex].value;
  url = "calendar.php?date=" + date;
  openURLWindow(url, 800, 600);
}