What would cause pi's system date and time to drift while plugged into a power source and not on the Internet?

Question

I have two Pi 3 running at the same time with my program being executed while both are plugged into a power source with current date and time and not connected to the Internet. Couple of days later I noticed that both pi's date and time drifted so much that it is behind by 1 or 2 days late and even the time is far off. Not only that my program, which is supposed to run 24/7, is shutdown. I am starting to think that is due date and time drifting. So, I would like to know what would cause pi system date and time to drift while it is plugged into a power source and not connected to the Internet.

Answer 1

If you shutdown the Pi, then restart it restores the time to that last saved, which it does periodically while running.

The "culprit" is fake-hwclock, although this is better than the default, which would set the time to 1970.

The Pi is designed to set its time from the internet using ntp - it has no RTC.

It is simple and cheap to add a RTC if you do want it to work without internet access.

dmesg -T | grep clock may give you some clue what the Pi is up to while booting.