JS calculates age based on birthday

function parseDate(str){
 if((/^\d{4}[\-\/\s+]\d{1,2}[\-\/\s+]\d{1,2}$/)){
  return new Date((/[\-\/\s+]/i,'/'));
 }
 else if((/^\d{8}$/)){
  return new Date((0,4)+'/'+(4,6)+'/'+(6));
 }
 else{
  return ('An error occurred in the time conversion!  ');
 }
}
function GetAgeByBrithday(birthday){
 var age=-1;
 var today=new Date();
 var todayYear=();
 var todayMonth=()+1;
 var todayDay=();
 var birthday=parseDate(birthday);
 if(parseDate(birthday)!='An error occurred in the time conversion!  ')
 {
 birthdayYear=();
 birthdayMonth=();
 birthdayDay=();
 if(todayYear-birthdayYear<0)
 {
  alert("Birth date selection is wrong!");
 }
 else
 {
  if(todayMonth*1-birthdayMonth*1<0)
  {
    age = (todayYear*1-birthdayYear*1)-1;
  }
  else
  {
    if(todayDay-birthdayDay>=0)
    {//alert(thisDay+'-'+brithd+"_ddd");
      age = (todayYear*1-birthdayYear*1);
    }
    else
    {
      age = (todayYear*1-birthdayYear*1)-1;
    }
  }
 }
 return age*1;
 }
 else
 {
 return -1;
 }
}