Convert a mat-datepicker into a String

I'm trying to have a reactive form in which there is some dates that are entered by the user using mat-datepicker. The problem I face is that the output of the mat-datepicker is a moment object containing the date. I want to get the date in a String (for example : "YYYY-MM-DD") and not an object.

Currently, the html template of my form component looks like that :

<form [formGroup]="myForm">
    <mat-form-field>
        <mat-label>Date d'expiration</mat-label>
        <input matInput [matDatepicker]="picker" formControlName="expireDate">
        <mat-datepicker-toggle matSuffix [for]="picker"></mat-datepicker-toggle>
        <mat-datepicker #picker></mat-datepicker>
    </mat-form-field>
</form>

And, my component looks like that :

import { Component, OnInit } from '@angular/core';
import { FormBuilder, FormGroup, Validators } from '@angular/forms'

declare var require: any
const moment = require('moment');
@Component({
  selector: 'app-form',
  templateUrl: './form.component.html',
  styleUrls: ['./form.component.css'],
})
export class FormComponent implements OnInit{
  myForm : FormGroup;
  constructor(private fb: FormBuilder) { }
  ngOnInit(): void {
    this.myForm =  this.fb.group({
      expireDate: [moment(),[
        Validators.required,
      ]],
    });
  }
}

Thanks in advance!

HTML

<form [formGroup]="myForm">
  <mat-form-field>
    <mat-label>Date d'expiration</mat-label>
    <input matInput [matDatepicker]="picker" formControlName="expireDate" (dateChange)="changeDatePicker()">
    <mat-datepicker-toggle matSuffix [for]="picker"></mat-datepicker-toggle>
    <mat-datepicker #picker></mat-datepicker>
  </mat-form-field>
</form>

TS

Assign value to expiredate in formgroup

 changeDatePicker(): any {
        this.myForm.value.expireDate = moment(this.myForm.value.expireDate).format('YYYY-MM-DD');
      }

• For Multiple Date Pickers

HTML

<form [formGroup]="myForm">
  <mat-form-field>
    <mat-label>Date d'expiration</mat-label>
    <input matInput [matDatepicker]="picker" formControlName="expireDate" (dateChange)="changeDatePicker()">
    <mat-datepicker-toggle matSuffix [for]="picker"></mat-datepicker-toggle>
    <mat-datepicker #picker></mat-datepicker>
  </mat-form-field>

  <mat-form-field>
    <mat-label>Date d'expiration</mat-label>
    <input matInput [matDatepicker]="picker1" formControlName="expireDate2" (dateChange)="changeDatePicker()">
    <mat-datepicker-toggle matSuffix [for]="picker1"></mat-datepicker-toggle>
    <mat-datepicker #picker1></mat-datepicker>
  </mat-form-field>
</form>

TS

myForm = this.fb.group({
    expireDate: ['', Validators.required],
    expireDate2: ['', Validators.required],
  });

changeDatePicker(): any {
    this.myForm.value.expireDate = (moment(this.myForm.value.expireDate).format('YYYY-MM-DD'));
    this.myForm.value.expireDate2 = (moment(this.myForm.value.expireDate2).format('YYYY-MM-DD'));
  }