As Ken mentioned, it is usually the battery on the mainboard, which is empty and therefore causes the Bios to loose buffered data and the real time clock not to run.
Also after changing this battery you will need to have set time and date in the Bios first and also repeat other adjustments made, which are different from the boards default settings (i.e. SATA controller mode or boot sequence).
Best greetings from Germany
Olaf