Sometimes you want to know when someone logs in to your machine when they shouldn't or when you started working on your machine.
The quickest way on osx is a simple
last | grep reboot
It should return a list of your login times
reboot ~ Wed Dec 4 09:48 reboot ~ Tue Dec 3 08:18 reboot ~ Mon Dec 2 08:00 reboot ~ Sun Dec 1 08:52 reboot ~ Sat Nov 30 10:16 reboot ~ Fri Nov 29 18:04 reboot ~ Thu Nov 28 18:41
Which will tell you when your machine was accessed and also acts as a handy indicator of when you start work each day.