指定日時でshutdown

# echo "/sbin/shutdown -h now" | at 00:00 04/16/2012
job 1 at 2012-04-16 00:00

めも、未検証
参考
http://www.nbu.ac.jp/~hamada/pukiwiki/index.php?linux%E9%96%A2%E9%80%A3%2F%E3%82%B7%E3%82%B9%E3%83%86%E3%83%A0%E7%AE%A1%E7%90%86%2F%E8%87%AA%E5%8B%95%E3%82%B7%E3%83%A3%E3%83%83%E3%83%88%E3%83%80%E3%82%A6%E3%83%B3