How compare data in two different time?

Question

I Append live data to numpy array. How compare data in two different time? Or how call data by second ago? Example data(now),data(50 second ago)

Answer 1

Can you explain more

Answer 2

I take stock market data from the web and put it in an array. I want to have a function that can be called based on the last second since the stock price was saved. price(second ago)