calculate the time elapsed between start and finish dates

test.xlsx
I need to calculate the time elapsed between start and finish dates. I've figured the TODAY function, as explained here but I'm referring solely to dates in cells. How to?

I'm an idiot. =2nd cell - 1st cell gives me a number of days, format as number, no decimal places, add 1,000 comma separators, and I've a number of days.

Thanks for reading, I hope I made you laugh at least.