Time between two IR beam sensors
You've tried to print an interval at the bottom of the setup() function, using finishtime before you have ever used it. Fortunately, finishtime will have been initialized to zero before the program runs so the the arithmetic result will be zero, though meaningless.

In loop(), you've never completed the interval calculation before you try to print it. 

If nothing at all is being printed, check your wiring and be sure your code is getting the triggers. You haven't showed your wiring, so we can't help you debug that, in case there is a problem with that also.