Should be easy to change, I've done it before with apple desktops, however it was some freakish custom battery that they had to order a replacement for.
Best bet, sad to say, is probably to take it into the genius bar at the Apple Store and ask them what kind of battery it takes and if it's user-servicable.
Totally off-topic, but I find it odd that a Mac would have 1986 as a time reference. I mean, times in Unix are all relative to 1st January, 1970 because that marks the beginning of the so-called \"Unix Epoch\" and DOS used to reset to 1980 because that's when the IBM PC and DOS were introduced. Following the same logic, I'd have thought a Mac would have reset to 1984 because that's when the first Macintosh was released...