diff --git a/_build/schema/versionx.mysql.schema.xml b/_build/schema/versionx.mysql.schema.xml
index 6879990..6acd39f 100644
--- a/_build/schema/versionx.mysql.schema.xml
+++ b/_build/schema/versionx.mysql.schema.xml
@@ -45,8 +45,8 @@
             Deltas have a start and end time, because a delta may be the result of different
             deltas that were merged.
             -->
-        <field key="time_start" dbtype="timestamp" phptype="timestamp" null="false" />
-        <field key="time_end" dbtype="timestamp" phptype="timestamp" null="false" />
+        <field key="time_start" dbtype="timestamp" phptype="timestamp" null="true" />
+        <field key="time_end" dbtype="timestamp" phptype="timestamp" null="true" />
 
         <index alias="principal_class" name="principal_class" primary="false" unique="false" type="BTREE">
             <column key="principal_class" length="" collation="A" null="false" />
diff --git a/core/components/versionx/model/versionx/mysql/vxdelta.map.inc.php b/core/components/versionx/model/versionx/mysql/vxdelta.map.inc.php
index 00e5a8d..e33e25b 100644
--- a/core/components/versionx/model/versionx/mysql/vxdelta.map.inc.php
+++ b/core/components/versionx/model/versionx/mysql/vxdelta.map.inc.php
@@ -84,13 +84,13 @@
     array (
       'dbtype' => 'timestamp',
       'phptype' => 'timestamp',
-      'null' => false,
+      'null' => true,
     ),
     'time_end' => 
     array (
       'dbtype' => 'timestamp',
       'phptype' => 'timestamp',
-      'null' => false,
+      'null' => true,
     ),
   ),
   'indexes' =>