The date on the certificate seems to get stuck at the day you finished Advanced 1.
Below is a mix of screenshots of my Spanish certificate and profile, taken today. In the circle you see the date, but it´s the date when I completed Advanced 1. I completed Advanced 2 a few days ago.
It´s like this for my other languages as well, with the date stuck on the day I completed Advanced 1.
The date should really just be the date of the day you download the certificate, as all the stats are current. This seems like a pretty easy and quick thing to fix. Since there is now an option to include your certificate in your LinkedIn profile, I think it matters that the certificate should be accurate.