The oil companies state that diesel fuel is good for 12 mths. By that, they mean that it doesn't degrade seriously in that time frame.
Over 12 mths old, you're looking at slow but steady fuel degradation - but the degradation is very dependent on how the fuel is stored.
Fuel kept at a constant temperature (as in below-ground tanks) will not degrade as rapidly as fuel kept in above ground tanks with wide temperature variations.
Wide temperature variations leads to a steady water build-up in the fuel via condensation. Water in fuel encourages microbiological growth (algae).
In addition, the more volatile ingredients of diesel that assist with prompt starting will tend to evaporate as fuel ages. Aged diesel develops gummy deposits that clog fuel injection equipment.
Older diesels with low injection pressures and large injector orifices, and accumulated wear, will accommodate aged diesel fuel better than the latest diesels - with their extremely high pressure injection, electronic controls, and very fine tolerances.
If your fuel is more than 12 mths old and has been stored above ground, you're well advised to filter it and blend it with newer diesel, to reduce any potential problems.