The problem is with the Date & Time Control Panel (around sys 7 to 8 era) which will not let you set the century of the year. It cycles from 1920 through to 2019.
The OS itself understands beyond 2020.
A solution is to set the date via some other method. Some Alarm clock utilities may offer this.
When we had the power off last month, my 9500 (7.6.1) with flat battery reset the date. Fortunately, it runs Retrospect 4 (server) which identified the oddity, and offered a method to set the date correctly.
My P475 (7.5.5) which eats batteries for breakfast, was also stuck in 1920 recently. Retrospect includes a 'sync time on client' feature, so was able to correct the time here too (remotely from the 9500).