Quoted from altan:
I got the old battery out and measured the voltage... it was pretty good. 2.97V or so. I'm suspicious this wasn't the cause for poor time. I'll see how the time is after a few days or so.
I'm wondering if a previous owner didn't replace the battery and bend the metal tab so it's not pushing down enough of the battery to make solid contact.
IIRC you have to replace the battery on the prismcard to keep the time correct.