I am counting rotations of a shaft and timing events that occur during that rotation. I wish to display the event timing as a percentage of ~1/4 of the rotation timing. This would involve recording the rotation time, dividing that number by 3.6 and dividing that number by the value of the event timer, then displaying that percentage on an LCD array.
What you are looking for is the millis() function. It returns the time that passed in milliseconds since the board power up.
With this function you can find the time delta, and achieve all the described calculations.