Get the day/time of last reboot of a Linux box

You can get the amount of time in days, hours and minutes via good 'ole uptime.

localhost [~] $ uptime
 08:14:06 up 39 days,  1:04,  1 user,  load average: 0.79, 1.25, 1.40

However if you want to find out the more human friendly, date and time version.

localhost [~] $ last reboot
reboot   system boot  2.6.32-279.5.2.e Sat Feb  2 06:10 - 08:13 (39+01:03)