While military grade diesel used to keep for years, the engines that ran it were also quite simple things - designed to work anywhere in the world on almost any kind of diesel. As automotive performance has increased and emissions standards tightened, modern diesel engines have become much fussier because they have to work at extremely high pressures. As such, we wouldn't recommend using a tank of fuel that's been stood that long - you'll do more harm than good, dragging up all kinds of sludge into the fuel lines, blocking the filter and damaging the engine. Potentially, it won't even be able to start the engine - so yes, use fresh fuel and take the proper steps to recommission the car.