Changed date format to DATE_RFC3399 for packages with git/hg repositories

src/Composer/Package/Locker.php

@@ -432,6 +432,6 @@ class Locker
             }
         }
 
-        return $datetime ? $datetime->format('Y-m-d H:i:s') : null;
+        return $datetime ? $datetime->format(DATE_RFC3339) : null;
     }
 }