I long wanted a MacOS ebook manager
This message from August 2008 shows I was aware of calibre at the time, but hadn't adopted it.
I was clearly
trying it out in early 2009
And by March 2009, I must have been using it myself,
as I was recommending it to others.
So I probably started using calibre in January 2009.
I can't use the date column in calibre to tell, because I use it for the date I bought a book. First one there is Jan 2002, so I'd been buying books for seven years before I started using calibre. (So my initial calibre library was around 1500 books!)
Sixteen years on I've added about another 3500.
Oh - any anyone who likes and uses calibre and can afford to should subscribe to Kovid's Patreon account.
https://www.patreon.com/c/kovidgoyal/