Javascript AngularJS-将计算值从函数添加到范围

我有两个datepicker字段，它们是使用AngularStrap datepicker指令实现的：

计算结果如下所示

<input type="text" name="numberOfDays" ng-model="numberOfDays" 
value="{{numberOfDays()}} disabled />

为天数差异表达式的更改设置侦听器，例如

HTML

简化天数计算从此处开始-

Plunker示例-
您能为这个问题创建一个Plunker吗？非常感谢您的帮助（Phil.）你将如何在ng repeat中使用此功能？
$scope.numberOfDays = function() {
    if ($scope.startDate != null && $scope.endDate != null) {
         var startDate = $scope.startDate;
         var endDate = $scope.endDate;

         var millisecondsPerDay = 1000 * 60 * 60 * 24;
         var millisBetween = endDate.getTime() - startDate.getTime();
         var days = millisBetween / millisecondsPerDay;

         return Math.floor(days);
    }
};

<input type="text" name="numberOfDays" ng-model="numberOfDays" 
value="{{numberOfDays()}} disabled />

<input type="text" name="numberOfDays" ng-model="numberOfDays" />

$scope.$watch(function() {
    return Math.floor(($scope.endDate - $scope.startDate) / 86400000);
}, function(days) {
    $scope.numberOfDays = days;
});