just recently I wanted to know when I installed linux on my x86 and to my disappointment I couldn't find the exact answer but got an answer that could satisfy my curiosity.

My initial stint with linux was with Redhat 9 in those days they haven't ventured into Enterprise edition.

One answer is rpm -qi basesystem. kairo.wordpress.com

but I was sure that I installed linux much before the date specified by rpm -qi

another option is

dumpe2fs on the root fs partition.

eg:- dumpe2fs -h /dev/sda1

yet another way was

ls -l /etc/*-release

for ubuntu it's lsb-release

for redhat redhat-release

* out of these I found dumpe2fs as satisfying.