title: Store 'time' in firebase each time user clicks a button
body: <p>I have a todo list with a 'Done' button. I want to store the time in firebase each time the user clicks the button. The button gets enabled after a while and when the user clicks the button again i want to store the time again and not overwrite the existing time. In my code, the time gets overwritten.</p>
  
  <p>index.html</p>
  
  <pre><code>&lt;ion-list class = "list"&gt;
      &lt;ion-item class = "has-header" ng-repeat="todo in data.todos" class = "item" ng-class="{purchased: todos.status == 'purchased'}"&gt;
       {{todo.title}}
        &lt;button  ng-disabled="todo.stopSpam" ng-click="doneItem(todo)"&gt;Done
        &lt;/button&gt;
  </code></pre>
  
  <p>app.js</p>
  
  <pre><code>$scope.doneItem = function(todo) {
  
    todo.stopSpam = true; 
    todo.timestamp = Date();
  
    $interval(callAtInterval, 30000);
  
    function callAtInterval(){
      todo.stopSpam = false;  
    }
  
  };
  </code></pre>