diff --git a/ganttzk/src/main/java/org/zkoss/ganttz/FunctionalityExposedForExtensions.java b/ganttzk/src/main/java/org/zkoss/ganttz/FunctionalityExposedForExtensions.java
index 2bfd4ac8e..e32fd0369 100644
--- a/ganttzk/src/main/java/org/zkoss/ganttz/FunctionalityExposedForExtensions.java
+++ b/ganttzk/src/main/java/org/zkoss/ganttz/FunctionalityExposedForExtensions.java
@@ -255,6 +255,10 @@ public class FunctionalityExposedForExtensions<T> implements IContext<T> {
     private void updateTimeTracker(List<Task> tasksCreated) {
         for (Task task : tasksCreated) {
             timeTracker.trackPosition(task);
+            if (task.isContainer()) {
+                TaskContainer container = (TaskContainer) task;
+                updateTimeTracker(container.getTasks());
+            }
         }
     }