diff --git a/libraries/src/Application/WebApplication.php b/libraries/src/Application/WebApplication.php
index 74d0658511c72..279e40fced2b9 100644
--- a/libraries/src/Application/WebApplication.php
+++ b/libraries/src/Application/WebApplication.php
@@ -530,7 +530,7 @@ protected function respond()
 			// Last modified.
 			if ($this->modifiedDate instanceof \JDate)
 			{
-				$this->setHeader('Last-Modified', $this->modifiedDate->format('D, d M Y H:i:s'));
+				$this->setHeader('Last-Modified', $this->modifiedDate->format('D, d M Y H:i:s', false, false) . ' GMT');
 			}
 		}