You can do it simply from your terminal window.
sudo shutdown now
to shutdown your machine NOW.
where hh:mm is the time on 24hr clock to shutdown at 7:50 you need to type
sudo shutdown hh:mm sudo shutdown 7:50.
where m is the minutes. If you type
sudo shutdown +m sudo shutdown +120
the machine will shutdown itself in 120 minutes.
And for the non terminal guys there is an application for doing this and more its called Gshutdown.
You can install Gshutdown by simply running the
sudo apt-get install gshutdown
command in your terminal or through Synaptic Package Manager.