Angular JSON using $scope data as key value

I have the following function which creates a new JSON object using data submitted through a form.

$scope.createEvent = function() {
  var cal = new CAL.API();
  cal.year = {August: [{day:$scope.calDay, title: $scope.calTitle, summary: $scope.calSummary, description: $scope.calDescrip}]};
  cal.$save(function(result){
    $scope.calendar.push(result);
  });
} 

I'd like to make the key, in this case the month August to be dynamic and pick up a $scope.calMonth field from the form. For some reason replacing the month with a dynamic $scope field doesn't seem to work. Is there any way to make this key value dynamic?

For reference here is my schema

year: { 
August: [
  {
    day: String,
    title: String,
    summary: String,
    description: String
  }
      ],
September: [
  {
    day: String,
    title: String,
    summary: String,
    description: String
  }
      ]
}
});

Here is my form:

<form name="calForm" ng-submit="createEvent()">
  <select ng-model="calMonth" required>
    <option>January</option>
    <option>February</option>
    <option>March</option>
    <option>April</option>
    <option>May</option>
    <option>June</option>
    <option>July</option>
    <option>August</option>
    <option>September</option>
    <option>October</option>
    <option>November</option>
    <option>December</option>
  </select>
  <input type="text" placeholder="Day" ng-model="calDay" required>
  <input type="text" placeholder="Title" ng-model="calTitle" required>
  <textarea type="text" placeholder="Summary" ng-model="calSummary" required></textarea>
  <br>
  <button class="addAdmin" type="submit">Add</button>
</form>