Back in the day, Unix wrote the time of day in the superblock for the root fs before unmounting it and rebooting.
sudo LC_ALL=C /sbin/tune2fs -l /dev/nvme0n1 |grep 2022 tune2fs 1.46.6-rc1 (12-Sep-2022) Last mount time: Thu Dec 29 20:05:22 2022 Last write time: Fri Dec 30 11:14:35 2022 Last checked: Wed Aug 31 09:37:57 2022
(It looks like "Last write time" is actually "Last umount time", because it is not refreshed during usage of the FS)