diff --git a/src/Services/LineFormatter.php b/src/Services/LineFormatter.php
index 32f464e..52b7a88 100644
--- a/src/Services/LineFormatter.php
+++ b/src/Services/LineFormatter.php
@@ -57,7 +57,9 @@ class LineFormatter implements LineFormatterInterface {
           break;
 
         case SyslogFormatEntry::TIMESTAMP:
-          $data = $this->dateFormatter->format($parsed_line[$syslog_format], 'short');
+          if (is_numeric($parsed_line[$syslog_format])) {
+            $data = $this->dateFormatter->format($parsed_line[$syslog_format], 'short');
+          }
           break;
 
         case SyslogFormatEntry::USER: