It may very well be the battery.
I think you should still be able to set the clock for the session, though.
try opening a CLI and doing a 'SETCLOCK RESET' (no quotes, of course) and see what happens. If it gurus, it's probably the battery or clock circuit. If it succeeds, it may still be, but it may also then let you set the time properly.
I seem to recall having to reset the clock on my Supra 512k trapdoor every once in a while, and I think sometimes the symptom would be the 500 guruing on the time tool.
