Fixed the GA calculator

Thanks to the folks who let me know that the GA calculator got broken. I think I’ve fixed the problem, but please let me know if you notice any problems.

Details: My web provider updated to a newer version of PHP, which broke the function I was using to generate the text of the difference between two dates -- e.g., “2 months and 5 days.” I found a nice built-in function that seems to do the same thing with less effort, so all is well. I think...