diff --git a/phpBB/includes/acp/acp_board.php b/phpBB/includes/acp/acp_board.php
index 4af9d52e44..937d9e926f 100644
--- a/phpBB/includes/acp/acp_board.php
+++ b/phpBB/includes/acp/acp_board.php
@@ -916,7 +916,14 @@ class acp_board
 
 		// Let the format_date function operate with the acp values
 		$old_tz = $user->timezone;
-		$user->timezone = new DateTimeZone($config['board_timezone']);
+		try
+		{
+			$user->timezone = new DateTimeZone($config['board_timezone']);
+		}
+		catch (Exception $e)
+		{
+			// If the board timezone is invalid, we just use the users timezone.
+		}
 
 		$dateformat_options = '';
 
diff --git a/phpBB/includes/user.php b/phpBB/includes/user.php
index 48c328214d..fcbfaaddfa 100644
--- a/phpBB/includes/user.php
+++ b/phpBB/includes/user.php
@@ -127,7 +127,15 @@ class phpbb_user extends phpbb_session
 			*/
 		}
 
-		$this->timezone = new DateTimeZone($user_timezone);
+		try
+		{
+			$this->timezone = new DateTimeZone($user_timezone);
+		}
+		catch (Exception $e)
+		{
+			// If the timezone the user has selected is invalid, we fall back to UTC.
+			$this->timezone = new DateTimeZone('UTC');
+		}
 
 		// We include common language file here to not load it every time a custom language file is included
 		$lang = &$this->lang;