Try using:
You can (ngModelChange) in your calender. it will trigger when value of the field get changes.
<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']"
(onSelect)="onChangeEndDate()" (ngModelChange)="onChangeEndDate()" [readonlyInput]="true"></p-calendar>
I think this will help you...
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