I have a late 2012 iMac. I use to shut it down with the shutdown command, in order to be able to delay the shutting down.

Here's how I use it:

sudo shutdown -h +30         # shuts down in 30 mins
sudo shutdown -h 04:00       # shuts down at the next 04:00 AM
sudo shutdown -h 1304302100  # shuts down on 04/30/2013 at 09:00 PM

I noticed that the latter two options don't work every time for me. Since I'm sure I entered the right time/date, what could prevent my Mac from shutting down? I set it to never go into stop, and to lock the screen after 30 mins of inactivity.

I'm on 10.8.3.