Activity End date through API

Hi All,

I need to get start date and end date of an activity. If the activity triggers multiple times in the process am able to get as many activities as triggered but while not able to fetch end date for all of them. From the below fig "Analyze change Request" activity can appear multiple times

This is the code am using

QueryResult qr = WorkflowHelper.service.getWorkItems(changeReqObj);

       while ( qr.hasMoreElements() ) {

       WorkItem workItem = (WorkItem) qr.nextElement();

       WfActivity activity = (WfActivity) workItem.getSource().getObject();

}