Angular update button on timer

Hi I'm trying to update a button with the elapsed time. I can console log the timer, and update the button label once, but it's not updating with the time

 <button ng-show="location"  class="button button-large assertive icon     {{buttonIcon}} button-block" ng-click="record()">
  {{buttonLabel}}
</button>

    $scope.buttonLabel = "stop";
    $scope.recordParams.timer = $interval(function(){
      var time = getTimeElapsed(initialTime);
      $scope.buttonLabel = time;
    }, 100);

i.e. it will change to 'stop' but I can't get it to update with the time.