Just had a first-time experience that I thought might be useful for people here.
I just received a very cool 5500 (actually, a 5500 board in a Color Classic, but that's irrelevant for this topic). Only problem was, the clock was off. Not only was it off, but the menubar showed 12:00AM, Jan 1, 1904, while the Date and Time Control Panel showed 6:28AM, February 6, 2040.
These are the earliest and latest dates+times, respectively, that a Mac of this era can display. But I'd never seen them displayed simultaneously!
In addition, whenever I reset the date and time, no amount of PRAM clearing or preference-trashing would make my changes "stick" after a restart.
Finally, I noticed that even through a PRAM reset, my custom mouse tracking and time zone settings would survive. And when I rebooted -- even after unplugging the computer from AC -- I never got the expected "Your Clock is set wrong" message when the desktop appeared.
I removed the PRAM battery and tested it. It read 2.2V -- about half the proper 4.5V (this is one of the square Rayovac ones, not the 1/2 AA ones).
Acting on a hunch -- and not having any full-strength batteries around -- I put a totally dead (less than 0.75V) battery in.
Lo and behold, the problem was fixed! I still have to reset the clock if I unplug the computer from AC of course. But the "Clock set wrong" alert appeared when I first booted after installing the battery, and now all my date and time settings "stick" properly.
So it seems that a half-dead PRAM battery can corrupt the PRAM and prevent the Mac from resetting the PRAM, or from recognizing that the battery isn't properly retaining PRAM settings.
Just an FYI.