I am trying to add days selected from dropdown. But the dates coming up after adding is way back in 1700s. What am I doing wrong here?

  selectedDays: number = 0;
  //event handler for the select element's change event
  selectDaystoadd (event: any) {
    //update the ui
    this.selectedDays = event.target.value;
    console.log(this.selectedDays);
  }

  formatDateField(event: Date, formControl: string) {
    this.form
      .get(formControl)
      .patchValue(
        this.datePipe.transform(event.getTime() + this.selectedDays, "yyyy-MM-dd'T'HH:mm:ss")
      );
      console.log(event);
  }

HTML code..

     <label>Date</label>
        <input id="input--payout-date"
          class="form-control"
          formControlName="payoutDate"
          [owlDateTime]="payoutDate"
          (dateTimeInput)="formatDateField($event.value, 'payoutDate')"
          [owlDateTimeTrigger]="payoutDate"
        />
        <owl-date-time [pickerType]="'calendar'" #payoutDate></owl-date-time>
        <label>Add days to date</label>
        <select (change)="selectDaystoadd($event)">
          <option value="0">0</option>
          <option value="20">+20</option>
          <option value="50">+30</option>
          <option value="90">+90</option>
        </select>