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.