[sjducb]

Diesel goes off fairly slowly. One year old diesel can be put into any modern car. Two year old diesel can be mixed 50/50 with fresh diesel and put into a modern car 10+ year old diesel can go into a tractor or an old (pre 2005) diesel car.

[pomatic]

Water ingress into stored diesel is calamitous. See also ‘diesel bug’.

[zrobotics]

Yeah, algae growth in diesel fuel can be even more catastrophic than putting stale gasoline into a fuel system. It can infect the rest of the system. A family member is a farmer, and 3 years ago had a semi develop algae growth in the fuel system. The tank needed to be removed and power washed, the entire fuel system needed to be extensively flushed. This ended up costing thousands.

In general, I wouldn't recommend storing fuel for the average person beyond a gallon for lawnmowers. It isn't worth the safety hazard, and the only way to actually save money is if you are using & purchasing enough that you can contract your fuel purchase (sign a contract to purchase $x by y date) for bulk delivery.