getMonth抛出错误的月份（81）-Angular6

我正在尝试获取日期的月份，但它一直向我显示："2018-81-26 07:00"..这是我用来设置所需日期的函数：

selectStartTime(time) {
      this.selectedTime = '' + time.time + ' ' + time.period;

      var date = new Date(this.start_at); // this shows good date: 
      //Wed Sep 26 2018 00:00:00 GMT+0200 (Central European Summer Time)

      var period = time.period;
      var hours = '';

// error occurs here:
      var year = date.getFullYear() + '-' + date.getMonth()+1 + '-' + date.getDate();
// year: 2018-81-26 07:00



      if (period === 'PM') {
          hours = '' + (time.value + 12) + ':00';
          this.campaignModel.starts_at = year + ' ' + hours;
      } else {
          hours = time.time;
          this.campaignModel.starts_at = year + ' ' + hours;
      }
    }

我究竟做错了什么？

只需更改以下代码

var year = date.getFullYear() + '-' + (date.getMonth()+1) + '-' + date.getDate();