如何使用kendo UI PHP在甘特图中添加持续时间列

I am working on Gantt Charts using Telerik Keno UI PHP. I need to show duration column in the gantt chart. Since the duration of a task can be easily calculated by its start and end dates so there is no need to store it in the database. Task object has a duration() method that we can use to get the duration of a task I have tried following code to do that

$durColumn = new \Kendo\UI\GanttColumn();
$durColumn->field("duration")
      ->title("Duration")
      ->width(300);

but it return a jquery function in the gantt view

function(){ var e=this.end, t=this.start; return e-t }

can anybody help me in that