we've needed countdown timers for our youth service and for new years. i pointed a video camera at the beta deck and recorded the LED timer directly into the NLeditor. i used a luma keying to key out the black background and then used video effects to adjust color and place the digital numbers where i wanted them; i put them on top of juice (for the youth) or in a graphic frame (for new years).
the timer shows minutes:seconds:frames, but nobody keeps track of frames vs. hundreths cause it just blinks real fast. the obvious drawback to this option is it takes 15 minutes to record a 15 minute countdown, and 15 minutes to run it to tape. [it also takes 15 minutes to play it back...
use it straight from the computer, or for preproduction, you can then just record the output and lumakey it onto your video background. or you could also render with other background colors and chroma key the text or the background for other effects; i.e. put a countdownclock in picture with moving juice inside the counter text. [im assuming your equipment can do this...]