[CentOS] Display a warning message at a certain time ?

Tue Feb 2 20:47:20 UTC 2010
Niki Kovacs <contact at kikinovak.net>

Hi,

I'm currently installing a CentOS 5 desktop as a public internet access 
point. The machine shuts down every day automatically at 22:30. Is there 
a way I can display a message in GNOME at 22:15 warning the user that 
the machine will shutdown in 15 minutes ?

Any suggestions ?

Niki Kovacs