The who command can be used to:

• View the users that are logged in
• Display when system was last rebooted (this article)

The who command with the -b or --boot option can be used to display the date and time the system was booted.

[root@server1 ~]# who --boot
system boot 2017-04-14 17:48

 Similarly, the uptime command can be used to show how long the system has been up.

[root@server1 ~]# uptime
02:37:41 up 1 day, 23:55, 1 user, load average: 0.05, 0.06, 0.14