diff --git a/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/order/SubcontractModel.java b/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/order/SubcontractModel.java
index 0aecdf9e3..972fb60d8 100644
--- a/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/order/SubcontractModel.java
+++ b/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/order/SubcontractModel.java
@@ -27,8 +27,10 @@ import org.navalplanner.business.common.exceptions.ValidationException;
 import org.navalplanner.business.externalcompanies.daos.IExternalCompanyDAO;
 import org.navalplanner.business.externalcompanies.entities.ExternalCompany;
 import org.navalplanner.business.planner.daos.ISubcontractedTaskDataDAO;
+import org.navalplanner.business.planner.entities.StartConstraintType;
 import org.navalplanner.business.planner.entities.SubcontractedTaskData;
 import org.navalplanner.business.planner.entities.Task;
+import org.navalplanner.business.planner.entities.TaskStartConstraint;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.context.annotation.Scope;
@@ -105,12 +107,22 @@ public class SubcontractModel implements ISubcontractModel {
                 }
 
                 task.removeAllResourceAllocations();
+                convertOnStartOnFixedDate(task);
             }
 
             recalculateTaskLength();
         }
     }
 
+    private void convertOnStartOnFixedDate(Task task) {
+        TaskStartConstraint taskConstraint = task.getStartConstraint();
+        if (taskConstraint.isValid(StartConstraintType.START_IN_FIXED_DATE,
+                task.getStartDate())) {
+            taskConstraint.update(StartConstraintType.START_IN_FIXED_DATE, task
+                    .getStartDate());
+        }
+    }
+
     private void recalculateTaskLength() {
         Date previousStartDate = ganttTask.getBeginDate();
         long previousLength = ganttTask.getLengthMilliseconds();
diff --git a/navalplanner-webapp/src/main/webapp/planner/order.zul b/navalplanner-webapp/src/main/webapp/planner/order.zul
index e30af0d51..fb9fc14cf 100644
--- a/navalplanner-webapp/src/main/webapp/planner/order.zul
+++ b/navalplanner-webapp/src/main/webapp/planner/order.zul
@@ -89,10 +89,12 @@
                         <row id="startConstraint">
                             <label value="${i18n:_('Constraint')}" />
                             <vbox>
-                                <combobox id="startConstraintTypes">
+                                <combobox id="startConstraintTypes"
+                                    disabled="@{editController.isSubcontractedAndIsTask}">
                                 </combobox>
                                 <datebox id="startConstraintDate" constraint ="no empty"
-                                    value="@{editController.startConstraintDate}" />
+                                    value="@{editController.startConstraintDate}"
+                                    disabled="@{editController.isSubcontractedAndIsTask}" />
                             </vbox>
                         </row>
                         <row id="subcontract" visible="@{editController.isTask}">