And Michael's right about the Numbers effect. I THINK you have to apply it to a solid. Leading or trailing zeros on the timecode setting may not go away... I forget which. Thus, you may have to cut a mask on a second solid and use it as a track matte to conceal the unwanted digits.
I hope you're not obsessed with to-the-frame timings, because this effect doesn't know about drop frame time code. But hey, if you're just counting minutes & seconds down, it's good enough.
Now, if any of the AE lingo I threw around above is confusing, that's the reason Michael gave you that link, and why I also included it. Considering the other applications you use, you could proceed through it quickly.
But don't ignore the basics in AE: those misguided individuals who think they can blow them off very quickly find themselves confused, frustrated and generally at a loss even on projects of mild complexity.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA