vue elementui 计算一个月后的时间

watch:{
 'form.startTime'(val) {
      console.log('startTime', val)
      if (val) {
        var month = new Date(val).getMonth() + 1
        var year = new Date(val).getFullYear()
        // console.log(month, '月')
        // console.log(year, '年')
        if (month == 2) {
          if (year % 4 == 0 && year % 100 > 0) {
            // console.log('闰年')
            this.form.endTime = parseTime(new Date(val).getTime() + (60 * 60 * 1000 * 24 * 29))
            return
          }
          if (year % 4 != 0) {
          // console.log(2022 % 100)
            // console.log('平年')
            this.form.endTime = parseTime(new Date(val).getTime() + (60 * 60 * 1000 * 24 * 28))
          }
          return
        }
        if (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) {
          // console.log('1 3 5 7 8 10 12')
          this.form.endTime = parseTime(new Date(val).getTime() + ((60 * 60 * 1000 * 24 * 30) + (60 * 60 * 1000 * 24)))
          return
        }
        if (month == 4 || month == 6 || month == 9 || month == 11) {
          // console.log('4 6 9 11')
          this.form.endTime = parseTime(new Date(val).getTime() + (60 * 60 * 1000 * 24 * 30))
          return
        }

        console.log('endTime', this.form.endTime)
      }
    },
}