Get Shift Fire Fighting ENFD

<!DOCTYPE html>
<html>
  <head>
    <script>
      var shift = getShift();

      document.addEventListener("DOMContentLoaded", function(event) {
        document.getElementById("shift").innerHTML = "Today is a " + shift;
      });

      function getShift(){
        var oneDay = 24*60*60*1000; // hours*minutes*seconds*milliseconds
        var ashift = new Date(2015,8,15);
        var bshift = new Date(2015,8,16);
        var cshift = new Date(2015,8,17);
        var today = new Date();
        today.setHours(0,0,0,0);

        ashift = Math.round(Math.abs((ashift.getTime() - today.getTime())/(oneDay)));
        bshift = Math.round(Math.abs((bshift.getTime() - today.getTime())/(oneDay)));
        cshift = Math.round(Math.abs((cshift.getTime() - today.getTime())/(oneDay)));
        //diffDays % 2;
        if(ashift % 3 === 0){
          return "A shift";
          }else if(bshift % 3 === 0){
          return "B shift";
          }else if(cshift % 3 === 0){
          return "C shift";
          }else{
          return "unkown shift";
        }

      }
    </script>
  </head>
  <body>
    <h1 id="shift"></h1>
  </body>
</html>