Yii2在gridview中计算两个日期之间的天数

            'class' => 'kartik\grid\FormulaColumn', 
            'header' => 'Days Work', 
            'value' => function ($model, $key, $index, $widget) { 
                $time = new \DateTime('now');
                $today = $time->format('Y-m-d');
                $p = compact('model', 'key', 'index');
                $datetime2 = $model->activedate;
                $interval = $today->diff($datetime2)->days;
                return $interval;
            },
            'headerOptions' => ['class' => 'kartik-sheet-style'],
            'hAlign' => 'right', 
            'width' => '7%',
        ],

我使用上面的代码来计算今天的日期和活动日期之间的日期。代码显示错误 Call to a member function diff() on string .

请告诉我哪里错了。

非常感谢。