p-calendar minimum validation issue in angular start and end date

I have used p-calendar in my angular laravel project and have to apply date range as start date and end date validation but If I choose start date then it is not applying on the end date as minimum validation then if I change month to previous or next then only the minimum validation is getting apply.

<p-calendar class="date"
(onSelect)="onChangeDate()"
 [(ngModel)]="model.start_date"
[minDate]="dateTime" [showIcon]="true"
[showTime]="true" showButtonBar="true"                                                      
[formControl]="form.controls['start_date']"
[readonlyInput]="true"></p-calendar>
</span>


<span>*End Date &nbsp;</span>
<p-calendar class="date"                                                                              
[minDate]="null!=model.start_date?model.start_date:dateTime"
[(ngModel)]="model.end_date"
[showIcon]="true" [showTime]="true"
showButtonBar="true"
[formControl]="form.controls['end_date']"
 [readonlyInput]="true"></p-calendar>
</span>

via Chebli Mohamed