diff --git a/program/include/main.inc b/program/include/main.inc
index 66de2e5d8..b0736625a 100644
--- a/program/include/main.inc
+++ b/program/include/main.inc
@@ -1062,15 +1062,25 @@ function format_date($date, $format=NULL, $convert=true)
   if (empty($ts))
     return '';
 
-  $date = new DateTime("@".$ts);
+  try {
+    $date = new DateTime("@".$ts);
+  }
+  catch (Exception $e) {
+    return 'unknown';
+  }
 
-  // convert to the right timezone
-  $stz = date_default_timezone_get();
-  $tz = new DateTimeZone($convert ? $RCMAIL->config->get('timezone') : 'GMT');
-  date_default_timezone_set($tz->getName());
-  $date->setTimezone($tz);
+  try {
+    // convert to the right timezone
+    $stz = date_default_timezone_get();
+    $tz = new DateTimeZone($convert ? $RCMAIL->config->get('timezones') : 'GMT');
+    $date->setTimezone($tz);
+    date_default_timezone_set($tz->getName());
 
-  $timestamp = $date->format('U');
+    $timestamp = $date->format('U');
+  }
+  catch (Exception $e) {
+    $timestamp = $ts;
+  }
 
   // define date format depending on current time
   if (!$format) {