I have a late 2012 iMac that runs Mountain Lion (10.8.3). I use to shut it down with the `shutdown` command, in order to be able to delay the shutting down. I also have disabled sleep entirely.

Here's how I use the shutdown command:

    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?

Again, I set it to **never** go into stop, and to lock the screen after 30 mins of inactivity. For example today my Mac shut down unexpectedly (where I am now it was 03:40 PM. I never set this time anywhere. Yesterday night I set it to `shutdown -h 04:00` and it was about 03:00AM.