If I change my system time on Debian which files would be modified?
Would it be /etc/default/rcS?
Also, is default time on Debian Dec 31 1969?
Current time information is not stored in a file, rather it's synced from your hardware clock with
The date Dec 31 1969 is the epoch date (representing this number in bits this would be all zeros), when your system resets the system clock, it is reset to the epoch date (all zeros).
How to fix the issue
And you're back in business. For continual syncing of the clock with an external source you should look at
If you would like to test your CMOS battery, the simplest way to do so is to shutdown your system, remove the power cord, wait for about 5 minutes (to remove stand-by power, and let the capacitors drain), then start everything back up. If your clock has reset again, get a new battery. You can usually find the correct battery at a drugstore. If this is a server, you should contact your vendor for a replacement (ASAP).
I'm not aware of any mechanism within Linux that would allow you to monitor the state of your CMOS battery.