Not sure if this works on iPhone too, but on Android, the way I could figure out the date exactly, was by looking at the date I got the “Complete tutorial” achievement - assuming you did this on the first day of play, which seems like it would normally be the case.
So on Android at least, you can find this in Options in the bottom left corner, just left of the sign in/sign out button for Google Play Games. It is interesting that these are the same accomplishments you can see in Play Games, but in there it does not seem to show the date.
Interestingly I first looked for this also when I wrote a post about looking back at my one year anniversary of play… which in 2 days will be 1 year ago. @myopia seems like you did quite well in one year, I would say… I’m at work and I haven’t had time to read your whole post yet in detail, but I will do that later.
Maybe I should do a 2 year write up in 2 days then… one thing I do remember that for me, I was sort of slow getting 5* in the beginning, and that accelerated a lot after the 1 year mark (spoiler alert, the vast majority from TC20 as I’ve been very much C2P). Will be interesting to compare then to now.