It's rather a cosmetic difference, but I want to fix our failures on MySQL engines suite (again), and this is one of them. So, if it's intended behavior, please reassign the bug back to me so I update the result file for engines/funcs/r/date_function.
MariaDB 5.3, 5.5, 10.0:
MySQL 5.1, 5.5, 5.6, MariaDB 5.2:
No warnings on either version.