If you did much dismantling of PCs back in the 1980s and ... dual-in-line devices containing clock and non-volatile RAM chips, a crystal, and a lithium battery. The battery was good for about ...

[RedruM69] recently ran into a system with a dead Real Time Clock (RTC) module ... and 100 or so bytes of battery backed SRAM (before the days of cheap, plentiful flash memory).