What in /var/logs shows system reboot?

[francis picabia <fpicabia@gmail.com>]

I'm running Debian 8.6, and looking at old logs.  I'd like to confirm when

the system was rebooted to invoke the newer kernel which fixed

the Dirty COW bug.  With systemd, I'm not seeing the old signs

like the announcement of the kernel version.  If I have a complete

copy of my /var/log from last October, which file will confirm

when the system was rebooted?  I've already tried last -f wtmp

and it didn't show anything.

This should be a simple question but I'm not seeing the usual

stuff like dmesg file activity in /var/log