"I have installed a linux machine 3 months back.
I want to know the exact date and time the machine was installed.
If you mean the "machine" yes the command above is a good indicator.
But if you mean the "date & time" three months ago that your "operating system" was "installed"... yes what about it? This thing mostly users doesn't care about, like me.
However try this:
$ ls -l /etc
and find out the date and time rc.d/ directory was created. Usually this folder is less touched and I used it to determine the date and time.
Hope this helps.