TLiu2014

  /**
   * An example of custom validator on a date string.
   * Valid date is a string, which is:
   * 1, in the form of YYYY-MM-DD
   * 2, later than today
   * 3, not an invalid value like 2018-20-81
   * @param date a date string
   */
  isMyDateFormat(date: string): string {
    if (date.length !== 10) {

TLiu2014

  getErrorMessage(pickerInput: string): string {
    if (!pickerInput || pickerInput === '' ) {
      return 'Please choose a date.';
    }
    return this.isMyDateFormat(pickerInput);
  }

TLiu2014

<form #datePickerForm>
  <mat-form-field>
    <input matInput [min]="minDate" [matDatepicker]="picker" placeholder="Choose a date" #pickerInput [formControl]="date">
    <mat-datepicker-toggle matSuffix [for]="picker"></mat-datepicker-toggle>
    <mat-datepicker #picker></mat-datepicker>
    <mat-error *ngIf="date.invalid">{{getErrorMessage(pickerInput.value)}}</mat-error>
  </mat-form-field>
</form>