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

Nice Job!

nice job
just needs:

1. get and set to convert value of empty string to/from valueAsDate of null
2. work in UTC date time
   therefore becomes something like

            Object.defineProperty(HTMLInputElement.prototype, "valueAsDate", {
                get: function(){
                    if (this.value === '') { return null; }
                    const d = this.value.split('-');
                    return new Date(Date.UTC(Number(d[0]), Number(d[1]) - 1, Number(d[2])));
                },
                set: function(d){
                    if (d === null) { 
                        this.value = '';
                        return;
                    }
                    const day = ('0' + d.getUTCDate()).slice(-2);
                    const month = ('0' + (d.getUTCMonth() + 1)).slice(-2);
                    this.value = d.getUTCFullYear() + '-' + month + '-' + day;
                }
            });