I have to point out that "justness" is not the same thing as "righteousness", for even a corrupt and wicked judge can occasionally dispense justice; Jesus told a parable to this effect.
The statement, "God is just" means that when He exercises judgment, it is always without partiality for any reason, and that He takes into account all relevant evidence. As an aside, He's also not bound by human legalese like a statute of limitations.
One of the key lessons (for me) from my seminary training was a brief exercise where we read a passage and then summarized what it said about God. Most of the students (myself included) extrapolated from the text. The professor gently corrected us, explaining that the text says
exactly what it says and no more. So in our studies, we do well to resist the temptation to claim the text says more (or different) than it actually does.