I think there should be a command in the game to see the date you joined the server. Something like the /seen command. I've looked around and can't find a way to find the date I joined your server and I'd really like to know because I've been with your server since 2013. Please consider this suggestion.
whatifwelikechangedourignlike6billiontimeswillitstillrecognizetheipthingermabober if it does support :3 butttttt ^^
I mean, it could be nice to have, though it's not really necessary. And as said above multiple times, what if the server resets?
I'm pretty positive the plugin would start recording joindates the moment it is installed, and not be able to see the ones dating back. So I'd say it does not work for your purpose, and only for new players. I don't often say this, but this time I REALLY do not see any reason in this.