It sounds as though your motherboard CMOS battery is dying. When you are prompted to go into setup at boot up it usually means there is a configuration problem. One thing you can try is to go into setup and check the date and time in the BIOS and change it if it's wrong. Then save and exit and see if that solves your problem.
Once you fix the configuration problem in the BIOS then that message should go away. If your time continues to be off then you should change the CMOS battery. They are only a couple of dollars and you can get them at an electronics store or even Amazon. Just be sure to bring the old one in to make sure you get the right one.