function setDays(tagid, monatid, jahrid) {
  var tag = document.getElementById(tagid).value;
  var monat = document.getElementById(monatid).value;
  var jahr = document.getElementById(jahrid).value;

  var februar = 28;

  if((jahr%4 == 0 && jahr%100 !=0) || jahr%400 == 0) {
    februar = 29;
  }

  var tage = new Array(31, februar, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);

  document.getElementById(tagid).options.length = tage[monat-1];

  for(i = 0; i < tage[monat-1]; i++) {
    document.getElementById(tagid).options[i].innerHTML = i+1;
    document.getElementById(tagid).options[i].value = i+1;
  }
}

function switchElem(box, id) {
  if(box.checked)
    document.getElementById(id).style.visibility = "visible";
  else
    document.getElementById(id).style.visibility = "hidden";
}