This article explains how the Energy Saver control panel works.

Stage 1

Fifteen minutes prior to shutdown, Energy Saver notifies the user that they scheduled the computer to shut down at a certain time. We then give them the option to cancel the shutdown, postpone it by 1 hour, or to simply go ahead and shut down.

Assuming the user is there, they make their selection and everything is fine. If the user is away from their computer, the dialog will dismiss itself (disappear from the screen with no action taken) after 5 minutes.

After this initial notification, which can be disabled under the Preferences menu, the computer then makes a best faith effort to determine if the user is actually done using the machine. If the computer is idle for 5 minutes, then Energy Saver assumes that you are done for the day.

Stage 2

Ten minutes prior to shutdown, the computer starts a vigil to wait for 5 minutes of idle time in one stretch. If the computer is idle for 5 minutes, it will assume that it can shut the computer down and moves on to stage 3.

Stage 3

After 5 minutes of idle, no sooner than 5 minutes prior to the scheduled shutdown time, the computer puts up a countdown dialog that informs the user of the impending shutdown. The user then has the option of cancelling the shutdown, or postponing it by 5 minutes (or more).

If the user chooses to cancel, nothing further happens.

If the user chooses to delay 5 minutes, Energy Saver simply reverts back to stage 2, waiting for a 5 minute idle window.

If the user does nothing, Energy Saver moves on to stage 4.

Stage 4

The Finder sends quit events to all open applications and shuts the computer down.

Now, the intention is that the user sees the first dialog and know the shutdown is coming. The second dialog is not really designed to be seen since presumably the user has gone.

There is one caveat that should be pointed out. If you are in Energy Saver, and you schedule a shutdown that is less than 15 minutes away, the Energy Saver control panel adjusts the times accordingly for TODAY only. For example, if it is 4:50 PM and you pick 5:00 PM, it shuts down at 5:06 PM today, but 5:00 PM from then on. This allows the proper staging of the above mentioned dialogs box messages.