I know on a Mac you can use a crontab. Here are the steps. I did get it to work, but on my new system I have had some problems, you can try it. You need to do this in the terminal.
sudo crontab -e
You'll need to enter your login password and then press Enter.
You'll now be in a text editor (vim). Carefully input the following keystrokes.
i once (this will allow you to enter text)
Let's say you want the machine to shutdown at 2am every day, we'd now enter:
* 2 * * * /sbin/shutdown -h now
If you wanted the machine to shutdown at 2:30am every day, you'd enter:
30 2 * * * /sbin/shutdown -h now
When you've typed this in, press esc
Then press shift+z shift+z (that's uppercase "z" twice, to write the changes and quit the editor).
You should now be dropped back to the command line where you started.