Some possible things that can cause the clock not to function correctly:

  • Dying/empty cmos battery.
  • Dying/broken motherboard or other attached hardware.
  • Installed software. Can be malware, but also legitimate software.

First thing I would do is replacing the battery.
A battery (CR2032) is only about $1 and you can replace one in +/- 10 minutes.
You may need to setup the bios again after doing so.
If you have to do so, depends on your system. (hardware)
Ofcourse first check if your system is using the CR2032 battery or another one.