// =================================== .component.html =====================

<h2>US Timezone Display</h2>
<div>
  <button [ngClass]="{'selected':('PST'==time_zone)}" (click)="onButtonClick3('PST')">PST</button>
  <button [ngClass]="{'selected':('MST'==time_zone)}" (click)="onButtonClick3('MST')">MST</button>
  <button [ngClass]="{'selected':('CST'==time_zone)}" (click)="onButtonClick3('CST')">CST</button>
  <button [ngClass]="{'selected':('EST'==time_zone)}" (click)="onButtonClick3('EST')">EST</button>
  <button [ngClass]="{'selected':('UTC'==time_zone)}"  (click)="onButtonClick3('clear')">Clear</button>
</div><br><br>
<h1>{{ time | date:'medium' :time_zone }}</h1>

// =================================== .component.ts =====================
  time_zone='PST'
  time = new Date();
  onButtonClick3(dt) {
    if(dt=="clear"){
      this.time_zone='PST'
    }else{
      this.time_zone=dt;
    }
  }

// =================================== .component.css =====================
 .selected {
   background-color: yellow;
 }