Calendar added

common/classes/Model.php

@@ -16,7 +16,7 @@ class Model {
                 $field = new \DateTime($field);
             }
 
-            if($field === false || $field->getTimestamp() == 0) {
+            if($field === false || $field->getTimestamp() <= 0) {
                 // If field had data but is invalid DateTime or is 0000-00-00 00:00, set it to null.
                 $field = null;
             } else {