获取最近三天，最近七天的时间

//获取近三天，近七天的时间
    getDay(day) {
      var today = new Date();
      var targetday_milliseconds = today.getTime() + 1000 * 60 * 60 * 24 * day;
      today.setTime(targetday_milliseconds); //注意，这行是关键代码
      var tYear = today.getFullYear();
      var tMonth = today.getMonth();
      var tDate = today.getDate();
      tMonth = this.doHandleMonth(tMonth + 1);
      tDate = this.doHandleMonth(tDate);
      return tYear + "-" + tMonth + "-" + tDate;
    },
    doHandleMonth(month) {
      var m = month;
      if (month.toString().length == 1) {
        m = "0" + month;
      }
      return m;
    }

 this.start_time = this.getDay(0) + " 23:59:59";//今天
 this.start_time = this.getDay(-3) + " 00:00:00";//近三天 
 this.start_time = this.getDay(-7) + " 00:00:00";//近七天