valueAsDate polyfill

valueAsDate.polyfill.js

if(!("valueAsDate" in HTMLInputElement.prototype)){
  Object.defineProperty(HTMLInputElement.prototype, "valueAsDate", {
    get: function(){
      var d = this.value.split(/\D/);
      return new Date(d[0], --d[1], d[2]);
    },
    set: function(d){
      var day = ("0" + d.getDate()).slice(-2),
          month = ("0" + (d.getMonth() + 1)).slice(-2),
          datestr = d.getFullYear()+"-"+month+"-"+day;
      this.value = datestr;
    }
  });
}

// test & demo: http://jsbin.com/wumikawero/1/edit?html,js,output