It is very easy to tell how long the system has been running using uptime command, but the information when exactly it was booted is not so apparent, so I will show you two different ways to get it.

The first way – the simplest one

Use last command to display the system shutdown entries and run level changes, limit output to the boot entries, and display only the first one.

$ last -x | grep boot | head -1
reboot   system boot  3.2.0-4-amd64    Sat Oct 19 12:44 - 23:24 (8+11:40)
System was booted at Sat Oct 19 12:44, current time is 23:24, and the uptime is 8 days, 11 hours, and 40 minutes.

The following examples demonstrate two simple ways to print the time when the system was booted, each one using different sets of commands.

$ last -x | awk '$3 ~ /boot/  {print $5 " " $6 " " $7 " " $8; exit}'
Sat Oct 19 12:44

$ last -x | grep -m 1 boot | tr -s ' ' | cut -d ' ' -f 5-8
Sat Oct 19 12:44

The second way – the more interesting one

Alternative solution is to read the /proc/uptime file, where the first number is the total number of seconds the system has been up and running.

$ cat /proc/uptime
735751.39 778420.68
System was up and running for the 735751 seconds which equals to 8 days, 12 hours, 22 minutes.

You can accurately determine when the system was booted using date command.

$ date --date "now - `cut -d ' ' -f 1 /proc/uptime` seconds"
Sat Oct 19 12:43:25 CEST 2013

The above-mentioned command construction shortly explains why I think that this solution is more interesting.