It is actually a lot easier then you think.
1) Create a new text track. You can type any random letters because we will change that anyway.
2) Twirl down the main text layer and highlight the text track itself in the timeline. This should bring up a different set of controls in the controls window, one of which is the generator tab.
3) For generator type, select ‘date/time’
4) Turn off all the params you don’t want like the year, month etc…
5) Make sure you deselect the ‘use current time’ checkbox.
6) Go to the frame where you want the time to start.
7) set the ‘numeric time’ parameter to 300 and make sure your interpolation is set to linear for realtime action. This should set you timeclock to 5:00 minutes because every number represents a second.
8) Go to 5 seconds later in your timeline and set the numeric time to 0.
Doing this should give you a 5 second countdown in realtime.
The numers I used for the example is a countdown from 5 seconds to 0. You can use any numbers you wish. Just play around and see what works best for you. Remember to set the 0 keyframe exactly 5 seconds ahead or you will not have a very accurate countdown.
Good Luck!