diff --git a/ganttzk/src/main/java/org/zkoss/ganttz/data/GanttDiagramGraph.java b/ganttzk/src/main/java/org/zkoss/ganttz/data/GanttDiagramGraph.java
index d2a5cebe2..54c67e4bb 100644
--- a/ganttzk/src/main/java/org/zkoss/ganttz/data/GanttDiagramGraph.java
+++ b/ganttzk/src/main/java/org/zkoss/ganttz/data/GanttDiagramGraph.java
@@ -1012,7 +1012,7 @@ public class GanttDiagramGraph<V, D extends IDependency<V>> implements
         private boolean taskChangesPosition() {
             ChangeTracker tracker = trackTaskChanges();
             Constraint.initialValue(noRestrictions())
-                    .withConstraints(new ForwardForces())
+                    .withConstraints(new BackwardsForces(), new ForwardForces())
                     .apply();
             return tracker.taskHasChanged();
         }
@@ -1055,6 +1055,12 @@ public class GanttDiagramGraph<V, D extends IDependency<V>> implements
                     ComparisonType.BIGGER_OR_EQUAL_THAN, start, end);
         }
 
+        DatesBasedPositionRestrictions lessThan(GanttDate start, GanttDate end) {
+            return new DatesBasedPositionRestrictions(
+                    ComparisonType.LESS_OR_EQUAL_THAN_RIGHT_FLOATING, start,
+                    end);
+        }
+
         class DatesBasedPositionRestrictions extends PositionRestrictions {
 
             private Constraint<GanttDate> startConstraint;
@@ -1270,6 +1276,28 @@ public class GanttDiagramGraph<V, D extends IDependency<V>> implements
 
         }
 
+        class BackwardsForces extends Forces {
+
+            @Override
+            PositionRestrictions enforceUsingPreviousRestrictions(
+                    PositionRestrictions restrictions) {
+                GanttDate result = Constraint.<GanttDate> initialValue(null)
+                        .withConstraints(restrictions.getEndConstraints())
+                        .withConstraints(adapter.getEndConstraintsFor(task))
+                        .apply();
+                if (result != null) {
+                    return moveToEnd(result);
+                }
+                return restrictions;
+            }
+
+            private PositionRestrictions moveToEnd(GanttDate newEnd) {
+                adapter.setEndDateFor(task, newEnd);
+                return lessThan(adapter.getStartDate(task), newEnd);
+            }
+
+        }
+
         @Override
         public int hashCode() {
             return new HashCodeBuilder()
diff --git a/ganttzk/src/main/java/org/zkoss/ganttz/data/constraint/ConstraintOnComparableValues.java b/ganttzk/src/main/java/org/zkoss/ganttz/data/constraint/ConstraintOnComparableValues.java
index 6a389dccc..23928ab07 100644
--- a/ganttzk/src/main/java/org/zkoss/ganttz/data/constraint/ConstraintOnComparableValues.java
+++ b/ganttzk/src/main/java/org/zkoss/ganttz/data/constraint/ConstraintOnComparableValues.java
@@ -54,7 +54,7 @@ public class ConstraintOnComparableValues<T extends Comparable<T>> extends
     }
 
     public enum ComparisonType {
-        LESS_OR_EQUAL_THAN, BIGGER_OR_EQUAL_THAN, EQUAL_TO;
+        LESS_OR_EQUAL_THAN, LESS_OR_EQUAL_THAN_RIGHT_FLOATING, BIGGER_OR_EQUAL_THAN, EQUAL_TO;
     }
 
     private final T comparisonValue;
@@ -75,6 +75,8 @@ public class ConstraintOnComparableValues<T extends Comparable<T>> extends
         switch (comparisonType) {
         case LESS_OR_EQUAL_THAN:
             return min(comparisonValue, value);
+        case LESS_OR_EQUAL_THAN_RIGHT_FLOATING:
+            return comparisonValue;
         case BIGGER_OR_EQUAL_THAN:
             return max(comparisonValue, value);
         case EQUAL_TO:
@@ -96,6 +98,7 @@ public class ConstraintOnComparableValues<T extends Comparable<T>> extends
     public boolean isSatisfiedBy(T value) {
         switch (comparisonType) {
         case LESS_OR_EQUAL_THAN:
+        case LESS_OR_EQUAL_THAN_RIGHT_FLOATING:
             return value.compareTo(comparisonValue) <= 0;
         case BIGGER_OR_EQUAL_THAN:
             return value.compareTo(comparisonValue) >= 0;