I have several Linux distributions installed on my PC (13 at last count) and I would like to know how I can find when I last booted each of them. Preferably as a list, like on one line when I last booted my Debian installation, one another line when I last booted my Gentoo install, on another line when I last booted my PCLinuxOS install, etc. I suppose I can check when the last edit to their files was made (as a way of testing when they were last started) but as I chroot into them regularly to update them (using their package manager) I thought this may not be an accurate way of checking.
It's a sure thing that each instance wrote more than one line of dmesg / syslog to
/var/log/* files at boot time. Filenames like
syslog are pretty popular, but YMWV. Mount the relevant partition RO, and go grep'ing for "kernel:" bootup messages. As a bonus, the syslog output will reveal the configured uname hostname of each instance. Looking for the string "Linux version" would be a good starting point, as it tends to appear early in the boot sequence.