last -x | head | tac

Examples of output:

runlevel (to lvl 0)   ... <-- first the system shuts down (init level 0)
reboot   system boot  ... <-- afterwards the system boots
runlevel (to lvl 2)   2.6.24-... Sat Sep 10 11:07 - 11:01 (2+23:34)
runlevel (to lvl 3)   ... <-- the system was running since this momemnt
reboot   system boot  ... <-- then we've a boot WITHOUT a prior shutdown
runlevel (to lvl 3)   3.10.0-693.21.1. Sun Jun 17 15:40 - 09:51  (18:11) 
Written by Matt Cooper
Hi, I'm Matt Cooper. I started this blog to pretty much act as a brain dump area for things I learn from day to day. You can contact me at: matt@linuxtutorial.co.uk.