I just tried to see Solr's "compiled on" date on my dev Solr server, which is running a 5.5.1-SNAPSHOT version, compiled from the branch_5_5 source.
The "impl" strings are so long that the only thing I could tell is that it was compiled this year. I will attach a screenshot.
It wasn't possible to see or highlight the full string. I did manage to see it all when I maximized my browser window (on a 1680x1050 screen), but I really dislike running with windows maximized. It's not like I keep the windows small either – the screenshot will reflect this.