First thing you'll want to do here is replace the CMOS battery - the system not keeping time is usually related to that battery going bad (it's what lets the computer keep time when power is off). I could not find any disassembly instructions for the model you list but you will need to get inside to swap out that battery (it is a small button battery).
Could be that replacing the battery cures all your issues, even if not, that's where to start
You are referring to the CMOS battery and not the main battery, correct? Did it take care of the "set clock" message? If the set system clock message is still appearing, I'll suggest trying a different battery for the CMOS since, at a minimum, that message should go away once the clock is set with a fresh battery - either that or your BIOS is corrupted, let's hope that isn't the case here