This gives you a 60 to 0 countdown.
In the Effect>Text>Numbers
Type: Numbers
Decimal Places: 0
then keyframe the Value/Offset/Random Max to 60, and another keyframe down the line to 0, and the countdown runs from 60 to 0 between keyframes.
Or
Maybe this earlier post will work, 60 is 1:00 then to 0:59, 0:58 etc. You need to set a simple rectangular mask to show only the 1:00, and add a slider (Effect>Expression DControls>Slider Control) so you can keyframe the countdown. Sam’s expression goes on the Source Text Stopwatch (Alt Click the stopwatch and Paste the expression), after you typed anything with the Text Tool. I like this one.
Name: sam.mltn
Date: May 22, 2006 at 1:18:53 pm
Subject: Re: Countdown Timer in AE
you can also use this expression with a slider to make a hrs:min:sec:1/100 timer bu masking out the top dot in the last column
timeToTimecode(t = effect(“Slider Control”)(“Slider”), timecodeBase = 100)
just took the basic expression from the global properties and changed the timecode base and added a slider to change the number.