Javascript Date age()

Date.prototype.age = function() {
 
 var birthday = new Date( this.getFullYear(), this.getMonth(), this.getDate() );
 
 var today = new Date();
 
 birthday = birthday.getTime();//from  w  ww  .  ja v a 2 s. c o  m
 today = today.getTime();
 
 var msYear = 1000*60*60*24*365.25;
 
 return Math.floor( (today - birthday) / msYear );
 
}

//var d = new Date(2008, 1);
//alert(d.numDays());