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

> 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 ?
>
cron

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