I was first introduced to this song by the parody of it on Futurama and then I heard the original on satellite radio and noticed that the year 6565 it jumps up a half step. And then it does it again in the year 9595. It seems as though the level of helium in the atmosphere increases every few thousand years. If you haven't seen the episode of Futurama, you have to catch it. I couldn't find a video but I included the audio link.
Zager & Evans: