I was 40 years off and less than 5km from the place and "only" got 7829. Assuming 99% accuracy for location, and based on some back-of-the-napkin math, that means you have to be within 70.7 years of the actual event to register any points for guessing the correct time. I think If you're within 100 years of the event you should get points. I think ideally it should be a curve, but if you can't program a curve then perhaps create brackets, like -10 points for each year within 10 years, -25 points for each year between 10 and 25 years, and -50 points for each year past that. Using this method you can be +/- 115 years of an event before getting 0 points for the time portion, and the closer you are, the closer to 5000 points you achieve.
Also, the one event was credited as being in Rome, although the picture shows, and the description says, St. Peter's Basilica, which is in Vatican City.