On Mac, like Linux and UNIX, you can use the
uptime command to see how long your computer has been on. Putting your computer to sleep does not reset this timer; it is time since boot.
But what if you want to see when your computer came out of sleep?
/usr/bin/pmset -g log | grep Wake | grep "due to" | tail -n1
Bam! Also tells you what woke the computer up.