How long has my Linux system been running?

uptime

If you want it in numerical form, it's the first number in /proc/uptime (in seconds), so the time of the last reboot is

date -d "$(</proc/uptime awk '{print $1}') seconds ago"

The uptime includes the time spent in a low-power state (standby, suspension or hibernation).

You can use uptime or last

To see only the last time

last reboot -F | head -1 | awk '{print $5,$6,$7,$8,$9}'

more generically

last reboot

Note and warning

The pseudo user reboot logs in each time the system is rebooted.  
Thus last reboot will show a log of all  reboots since the log file was created.

I usually use who -b, which produces output such as:

$ who -b
         system boot  2014-05-06 22:47
$

It tells me the date and time when the machine was last booted, rather than the time that has elapsed since it was last booted.

This command works on many other Unix systems too (Solaris, …).