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 7501d3f3f..f62e43d27 100644
--- a/ganttzk/src/main/java/org/zkoss/ganttz/data/GanttDiagramGraph.java
+++ b/ganttzk/src/main/java/org/zkoss/ganttz/data/GanttDiagramGraph.java
@@ -1039,6 +1039,9 @@ public class GanttDiagramGraph<V, D extends IDependency<V>> implements
                     .withConstraints(currentLength)
                     .withConstraints(adapter.getEndConstraintsGivenIncoming(incoming))
                     .apply();
+            if (newEnd == null) {
+                return false;
+            }
             if (hasChanged(previousEndDate, newEnd)) {
                 adapter.setEndDateFor(task, newEnd);
             }
@@ -1059,13 +1062,14 @@ public class GanttDiagramGraph<V, D extends IDependency<V>> implements
         }
 
         private boolean hasChanged(GanttDate old, GanttDate newValue) {
-            if (old == newValue) {
-                return false;
-            }
-            return (old == null) != (newValue == null)
-                    || old.compareTo(newValue) != 0;
+            return old != newValue && old.compareTo(newValue) != 0;
         }
 
+        /**
+         * Calculates the new start date based on the present constraints. If
+         * there are no constraints this method will return the existent start
+         * date value.
+         */
         private GanttDate calculateStartDateFor(V task, Set<D> withDependencies) {
             List<Constraint<GanttDate>> dependencyConstraints = adapter
                     .getStartConstraintsGiven(withDependencies);
@@ -1082,6 +1086,9 @@ public class GanttDiagramGraph<V, D extends IDependency<V>> implements
                         .withConstraints(adapter.getStartConstraintsFor(task))
                         .withConstraints(globalStartConstraints).apply();
             }
+            if (newStart == null) {
+                return adapter.getStartDate(task);
+            }
             return newStart;
         }
 
diff --git a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/TaskElement.java b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/TaskElement.java
index b5c98705a..38807bfb5 100644
--- a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/TaskElement.java
+++ b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/TaskElement.java
@@ -35,6 +35,8 @@ import java.util.SortedMap;
 import java.util.TreeMap;
 
 import org.apache.commons.lang.Validate;
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
 import org.hibernate.validator.NotNull;
 import org.joda.time.LocalDate;
 import org.navalplanner.business.calendars.entities.BaseCalendar;
@@ -54,6 +56,8 @@ import org.navalplanner.business.workingday.ResourcesPerDay;
  */
 public abstract class TaskElement extends BaseEntity {
 
+    private static final Log LOG = LogFactory.getLog(TaskElement.class);
+
     public interface IDatesInterceptor {
         public void setStartDate(IntraDayDate previousStart,
                 IntraDayDate previousEnd, IntraDayDate newStart);
@@ -251,6 +255,9 @@ public abstract class TaskElement extends BaseEntity {
     }
 
     public void setIntraDayStartDate(IntraDayDate startDate) {
+        if (startDate == null) {
+            LOG.error(doNotProvideNullsDiscouragingMessage());
+        }
         IntraDayDate previousStart = getIntraDayStartDate();
         IntraDayDate previousEnd = getIntraDayEndDate();
         this.startDate = startDate;
@@ -281,11 +288,21 @@ public abstract class TaskElement extends BaseEntity {
     }
 
     public void setIntraDayEndDate(IntraDayDate endDate) {
+        if (endDate == null) {
+            LOG.error(doNotProvideNullsDiscouragingMessage());
+        }
         IntraDayDate previousEnd = getIntraDayEndDate();
         this.endDate = endDate;
         datesInterceptor.setNewEnd(previousEnd, this.endDate);
     }
 
+    private String doNotProvideNullsDiscouragingMessage() {
+        return "The provided date shouldn't be null.\n"
+                + "Providing null values to start or end dates is not safe.\n"
+                + "In a near future an exception will be thrown if you provide a null value to a start or end date.\n"
+                + "Please detect the caller and fix it";
+    }
+
     public IntraDayDate getIntraDayEndDate() {
         return endDate;
     }
@@ -304,33 +321,27 @@ public abstract class TaskElement extends BaseEntity {
     }
 
     private IDatesHandler getDatesHandler(Scenario scenario) {
-        return ignoreNullDates(createDatesHandler(scenario));
+        return noNullDates(createDatesHandler(scenario));
     }
 
-    private IDatesHandler ignoreNullDates(final IDatesHandler decorated) {
+    private IDatesHandler noNullDates(final IDatesHandler decorated) {
         return new IDatesHandler() {
 
             @Override
             public void resizeTo(IntraDayDate endDate) {
-                if (endDate == null) {
-                    return;
-                }
+                Validate.notNull(endDate);
                 decorated.resizeTo(endDate);
             }
 
             @Override
             public void moveTo(IntraDayDate newStartDate) {
-                if (newStartDate == null) {
-                    return;
-                }
+                Validate.notNull(newStartDate);
                 decorated.moveTo(newStartDate);
             }
 
             @Override
             public void moveEndTo(IntraDayDate newEnd) {
-                if (newEnd == null) {
-                    return;
-                }
+                Validate.notNull(newEnd);
                 decorated.moveEndTo(newEnd);
             }
         };