I would argue that for c# this is more like a 7kyu, however the JS version is significantly harder as you have to handle any type of input.

I know I've been paying attention to what I'm doing and improving my linq when I solve a kyu and my solution is virtually identical to the most voted one :P.

Also is it just me or does it seem like many of the 6 kyu can be solved with good linq one-liners, but as soon as jump up to 5 kyu that becomes much more difficult?

I'm guessing my errors are coming from the fact that the random tests are including years > 9999. I feel its only fair, especially considering 8s are usually geared towards new folx, that you either include this fact in the description as an edge case, or remove tests above that threshhold.