downtimed is a program that monitors operating system downtime, uptime,
shutdowns and crashes and keeps records of those events.

downtimed(8) is a daemon process which is intended to be started
automatically from system boot scripts every time when the operating
system of a server starts. First the daemon logs its findings about the
previous downtime to a specified logging destination as well as in a
database file which can be displayed with downtimes(1). After that the
daemon just keeps waiting in the background and periodically updates
a time stamp file on the disk.

downtimes(1) is a command-line tool which can be used to inspect previous
downtime records.