diff --git a/src/Monolog/Handler/RotatingFileHandler.php b/src/Monolog/Handler/RotatingFileHandler.php
index cd938057..c4b2227a 100644
--- a/src/Monolog/Handler/RotatingFileHandler.php
+++ b/src/Monolog/Handler/RotatingFileHandler.php
@@ -68,11 +68,12 @@ class RotatingFileHandler extends StreamHandler
 
     public function setFilenameFormat($filenameFormat, $dateFormat)
     {
-        if (!in_array($dateFormat, array(self::FILE_PER_DAY, self::FILE_PER_MONTH, self::FILE_PER_YEAR))) {
+        if (!preg_match('{^Y(([/_.-]m)([/_.-]d)?)?$}', $dateFormat)) {
             trigger_error(
-                'Invalid date format - format should be one of '.
-                'RotatingFileHandler::FILE_PER_DAY, RotatingFileHandler::FILE_PER_MONTH '.
-                'or RotatingFileHandler::FILE_PER_YEAR.',
+                'Invalid date format - format must be one of '.
+                'RotatingFileHandler::FILE_PER_DAY ("Y-m-d"), RotatingFileHandler::FILE_PER_MONTH ("Y-m")'.
+                'or RotatingFileHandler::FILE_PER_YEAR ("Y"), or you can set one of the '.
+                'date formats using slashes, underscores and/or dots instead of dashes.',
                 E_USER_DEPRECATED
             );
         }