Angular Material - Override DateAdapter and change format of date picker fields

Create AppDateAdapter.ts file

import { Inject, Injectable, Optional } from '@angular/core';
import { DateAdapter, NativeDateAdapter } from '@angular/material';

export class AppDateAdapter extends NativeDateAdapter {
    parse(value: any): any {
        if (value) {
            const timestamp = typeof value === 'number' ? value : Date.parse(value);
            let date_regex = /([0-9]{2}[0-9]{1}\d{1})-([0]{1}[1-9]{1}|[1]{1}[0-2]{1})-([0]{1}[1-9]{1}|[12]{1}\d{1}|[3]{1}[01]{1})/; // Can you own regex or remove it.
            if (date_regex.test(value)) {
                return isNaN(timestamp) ? null : new Date(timestamp);
            } else {
                return new Date(undefined);
            }
        }
        else {
            return;
        }

    }
    format(date: Date, displayFormat: Object): string {
        if (displayFormat == "input") {
            let day = date.getDate();
            let month = date.getMonth() + 1;
            let year = date.getFullYear();
            let finalDate = `${year}-${this._to2digit(month)}-${this._to2digit(day)}`; // FORMAT IS YYYY-MM-DD  while default format is mm/dd/yyyy
            return finalDate;
        } else {

            return date.toDateString();
        }
    }

    private _to2digit(n: number) {
        return ('00' + n).slice(-2);
    }
}


export const APP_DATE_FORMATS =
    {
        parse: {
            dateInput: { month: 'short', year: 'numeric', day: 'numeric' }
        },
        display: {
            //dateInput: { month: 'short', year: 'numeric', day: 'numeric' },
            dateInput: 'input',
            monthYearLabel: { month: 'short', year: 'numeric', day: 'numeric' },
            dateA11yLabel: { year: 'numeric', month: 'long', day: 'numeric' },
            monthYearA11yLabel: { year: 'short', month: 'long' },
        }
    }

Update app.modules.ts file

import  DateAdapter, NativeDateAdapter, MD_DATE_FORMATS } from '@angular/material';
import { AppDateAdapter, APP_DATE_FORMATS } from "./Common/app-date-adapter"; // your file


 providers: [
   {
      provide: DateAdapter, useClass: AppDateAdapter
   } 
]

parse(value: any): any - it is not accepting string as return value. it either accepts null or date object. Is there anyway to return as string?