Hardware clock: BIOS clock unstable.

Hi: I turn off the machine. I turn it on set the BIOS clock to the correct time. Then I boot Debian 10 and the date linux command shows the system time is exactly = correct time + 3 hours. If instead I choose to boot Slackware -current the same happens. The difference between current time and system time is exactly 3 hours.

What is more, when I turn off the machine and enter the BIOS again, I find the clock is set to correct time + 3 again! At first sight it seems as if the problem is in the BIOS itself. How can this be? I think I had never before observed this behavior.