Kbase Article of the Week: PowerBook 100 through PowerBook 5300: Resetting Power Management Unit »

Apple Support:

This document describes the PowerBook Power Manager and explains how to reset it. Resetting the power manager also resets the date and time. After the reset, on some PowerBooks, the system clock is set to 00:00 on 1 January 1904.

This document refers to Apple products manufactured before November 1997. Click here for products manufactured after that date.

Naturally, that link is broken, but the Internet Archive grabbed it.