// vergangene Tage seit meinem Geburtstag

function addCommas( strValue ) {
var objRegExp = new RegExp('(-?[0-9]+)([0-9]{3})'); 

while(objRegExp.test(strValue)) {
strValue = strValue.replace(objRegExp, '$1,$2');
}
return strValue;
}

var birthday =new Date(1978, 1, 21) 
today=new Date()
var one_day=1000*60*60*24;
var days_since=Math.ceil((today.getTime()-birthday.getTime())/(one_day))

document.write(days_since.toString());