I am one of those who notes everything on a calendar, which later I will talk about the application I use for my list of things to do, upcoming events so as not to forget them (like my mother-in-law's birthday, or things like that HAHA), and keep something similar to a "log" of my life HAHA.
It happens that some time ago I needed to know the exact date when I installed my system, a simple command will tell us 
ls -lct /etc | tail -1 | awk '{print $6, $7, $8}'
It returns the following to me:
Nov 7 10:33
Which implies that my Arch I installed it on November 7th 
What it does is something simple, look in our folder / etc / the oldest file, and shows us the date of it.
Greetings and ... let's see, When did you install your system? 
Read before in: GnomeTips