onTaskEndDrag event is fired when user finished dragging a Task item, and only if user has dragged the task to a new location.
One parameter is passed to the handler:
ganttChartControl.attachEvent("onTaskEndDrag", function(task){ if (task.getId()==13) { // Move another related task to the same location ganttChartControl.getProjectById(1).getTaskById(2).setEST(task.getEST()); } else alert("New EST = " + task.getEST()); });