Need help with starting point in calculation project

Hello,

Here's what I need:

Data will be ascertained by turning a switch on and off. I need to record this time, accurate to the thousandth of a second, make a calcuation on this data, and display the post-calculation output data on a screen. A 7 seg screen would be fine, no need for anything fancy. Any idea on the simplest/cheapest way to do this?

Thanks much,

Ryan

Check out my Pinewood Derby timer, on my website. It measures to 9.99 seconds but by using a crystal 10X higher in frequency, it would measure to .999 seconds.