if you click on file while you are installing mac os you will have access to the installer post a screen shot of the log with wich mac osx version you are installing...
her is mine with mac osx v10.3
I would hazard that the date is akin to DOS's birthday which is January 1st 1980. Some machines revert back to the time the BIOS was first initialised during manufacture, for example January 1st 2003 for a Dell Axim X5.