js中怎么判断两个日期的大小

日期

    this.PlanEndTime = "2018/9/10"
    this.PlanStartTime = "2018/12/20"
    const _PlanEndTime = new Date(this.PlanEndTime)
    const _PlanStartTime = new Date(this.PlanStartTime)
    console.log(_PlanEndTime.getTime()-_PlanStartTime.getTime())

    if(_PlanEndTime.getTime() < _PlanStartTime.getTime()){
      this.toastTip("开始时间不能大于结束时间!")
      return false;
    }