Ion-datetime binding through ngModel ionic-v4

i am binding through ngModel. but variable returning always undefined. i am using ionStyle event.

<ion-item class="itemdiv">
        <ion-label class="modtitle" position="floating">First Contact</ion-label>
        <ion-datetime display-format="DD/MM/YYYY" id="dat" [(ngModel)]="myDate" (ionStyle)="showdate()"></ion-datetime>
      </ion-item>

export class ModaldetailPage{
public myDate;

constructor( ) { }

showdate(){
    console.log(this.myDate);
  }

when i m changing variable to:

public myDate = new Date().toISOString();

then always getting current date. not changed value by picker.

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