Very simple LED chaser

I have a VERY simple led chaser light that slows down in proportion to the time after a button is pressed. If the button is low then the circuit acts like a typical chaser light. I googled my question with no luck. Whats the command I am looking for to start a timer and get the elapsed time? Please take a look at the file attached below

Thanks, Kcoer

Strobe_Program.ino (2.23 KB)

start timer with "unsigned long start=millis();"
Elapsed time is calculated as "millis()-start";