js中将日期转化为秒数

<script>
function submitmouseover()
{
    var start=new Date(document.getElementById("startTime").value);
    var end=new Date(document.getElementById("endTime").value)
    if(start.getTime()>end.getTime()){
        alert("startTime is later than endTime!")
    }
}
</script>

 <form>
           <div>
                 <input id="startTime" name="startTime" type="datetime-local" name="startTime" class="form-control" />
                 <input id="endTime" name="endTime" type="datetime-local" name="endTime"  class="form-control" />
           </div>
           <button class="btn btn-primary" type="submit" id="submit" οnmοuseοver="submitmouseover()">提交</button>
  </form>