You don’t need expressions to do it. Its really pretty easy in AE.
1. Create a new comp the size and length you need (i.e. 5min)
2. Create a new solid, any color.
3. Go to Effects/Text/Numbers.
4. When the Numbers Properties Dialog Box comes up, choose your font and
alignment settings then click OK.
5. Then in the Numbers Effects Controls, under Format/Type choose
Timecode [30].
6. Then choose your color settings under Fill and Stroke.
7. Adjust the size if you need to.
8. Uncheck Proportional Spacing. This prevents the numbers from
bouncing around as they adjust to the different sizes of numbers.
9. Lastly, select your solid layer in the timeline and hit Ctrl + Alt +
R to reverse the layer and play the numbers backwards.
10. If you’d like you can Pre-Comp the whole thing and then mask off
the numbers you don’t need (i.e. hours and frames, unless you want
those).
Thats it. Pretty simple really. Hope this helps!
Good Luck!
BIOZHANG