var awaitingPicking = [
  {
   "id": "2452",
   "OrderLineGroups": [
     {
       "CollectionDetails": {
         "CollectionFrom": "2017-03-21T13:00:00.317"
       }
     }
   ]
  },
{
   "id": "2454",
   "OrderLineGroups": [
     {
       "CollectionDetails": {
         "CollectionFrom": "2017-03-24T12:55:00.317"
       }
     }
   ]
  }
 ]

for(var i = 0; i < awaitingPicking.length; i++){
  var a = moment(awaitingPicking[i].OrderLineGroups[0].CollectionDetails.CollectionFrom);
  var b = moment(new Date())
   this.thirtyMins = a.diff(b, 'minutes');
  console.log(this.thirtyMins, a.diff(b, 'minutes') + 'minute(s) left');
}

<div *ngFor="let prepare of awaitingPicking" id="prepareOrder">
   <button [ngClass]="{'halfHour':thirtyMins < 60, 'moreThanHour': thirtyMins > 60}"> {{ prepare.Id }}</button>
</div>