Go to File > New > Transparent Video and generate a new Transparent Video Clip.
Place the Transparent Video clip on the timeline on a track above the video you wish to add the countdown timer, for the duration of that clip.
Add the Timecode Effect, and with that Transparent Video clip selected on the timeline, in the Effect Controls Pane configure the Timecode display as to how you want it to look.
Once you have it set, right click on the Transparent Video clip in the timeline and choose Nest from the drop down menu.
Once you have created the Nest (the clip on the timeline will have turned green) right click again, this time on the Nest, and from the drop down menu choose Speed/Duration. Set it to run in reverse.
Now the timecode should be counting down.
Depending on the horsepower of your computer, you may need to render the effect to see it run smoothly.
MtD