How to see total uptime on a mac
The command you want is called uptime:

|ruby-2.1.2| cortana in ~
○ → uptime
14:49  up 3 days, 21:51, 4 users, load averages: 1.41 1.32 1.30


From the man page:


  The uptime utility displays the current time, the length of time the system has been up, the number of users, and the load average of the system over the last 1, 5, and 15 minutes.