@firstname.lastname@example.org I was curious about this too, and here’s what I found for Linux. I’d suppose there are equivalents in OpenBSD?
sudo tune2fs -l BLOCK_DEVICE | grep 'Filesystem created:'on a BLOCK_DEVICE whose filesystem was created at 1st machine use
smartctl -a BLOCK_DEVICE | grep Power_On_Hoursto check the total power-on hours of some BLOCK_DEVICE that’s been up since the machine’s 1st use
Obviously these both depend on having a block device (disk drive usu) whose life span is close to the machine’s total uptime. There are utilities like
tuptime in Linux, which I think are also compileable on OpenBSD, that you can install when you first start up a machine to keep this cumulative uptime but that doesn’t help after the fact unless you solve time travel!