What is the best way to calculate each workers earnings that start and end at random times and periods of times.

I'm creating a program that users can mine to and earn different type of rewards based on how much each of them earn on the same wallet ect.

Example: Ethermine allows you to have 100,000 workers. How would I calculate how much each worker has earned from the time they started running and ended?

Not sure if this goes here.