Using last command to show listing of last reboot / shutdown date and time. Last command which searches back through the file /var/log/wtmp (or the file designated by the -f flag) and displays a list of all users logged in (and out) since that file was created. Names of users and tty’s can be given, in which case last will show only those entries matching the arguments. Names of ttys can be abbreviated, thus last 0 is the same as last tty0.
To show listing of last reboot, type the following command
# last reboot
reboot system boot 2.6.18-194.32.1. Wed May 25 09:52 (90+21:57) reboot system boot 2.6.18-194.32.1. Sat Apr 23 09:20 (122+22:28) reboot system boot 2.6.18-194.32.1. Mon Apr 18 14:13 (4+02:33) wtmp begins Tue Apr 5 15:15:41 2011
To show listing of last shutdown, type the following command
# last -x | grep down
shutdown system down 2.6.18-194.32.1. Fri Apr 22 16:46 - 07:50 (123+15:03)
-x: Display the system shutdown entries and run level changes.