ion-datetime Can't force user to select a date

Question

I am trying to force the user to select a date. While by default ion-datetime displays the current date and time but if not selected it shows a date from 1966. How do I force the user select a date and time?

<ion-datetime-button datetime="datetime"></ion-datetime-button>
<ion-modal [keepContentsMounted]="true">
  <ng-template>
    <ion-datetime id="datetime"  [(ngModel)]="startdate" [showDefaultButtons]="true" color="primary"><span slot="title">Select Start Date</span></ion-datetime>
  </ng-template>
</ion-modal>