This shouldn’t need another expression. The expression you have is purely for formatting the required number. Assuming that you are now using the point control that I suggested, you just need to add keyframes at the various points to ramp up through time to suit.
So if I assume that your animation is over say 10 seconds, then at the beginning, set the value of the x portion of the point control to 12, add a keyframe. Then say at 5 seconds add another keyframe by changing the value to 17000, then say at 8 seconds set another keyframe with the value 4200,000,000.
Clearly you will need to play with timings and possibly keyframe interpolation, to get the ramping how you want it.
Declan Smith
https://www.madpanic.tv
After Effects CS5.5/ FCS3 / Canon 7D / Canon XL2 / Reason / Cubase