diff --git a/library/Icingadb/Model/Behavior/Timestamp.php b/library/Icingadb/Model/Behavior/Timestamp.php
index 3a624d5d..b29b3d62 100644
--- a/library/Icingadb/Model/Behavior/Timestamp.php
+++ b/library/Icingadb/Model/Behavior/Timestamp.php
@@ -24,7 +24,12 @@ class Timestamp extends PropertyBehavior
         }
 
         if (! ctype_digit($value)) {
-            $value = strtotime($value);
+            $timestamp = strtotime($value);
+            if ($timestamp === false) {
+                return $value;
+            } else {
+                $value = $timestamp;
+            }
         }
 
         return $value * 1000.0;