As Carbonation says, diesel fuel is a commodity but it's a regional one. They don't send winter-grade diesel to Florida or even southern California.
And the changeover date varies with the climate of the region. So if you are buying and using diesel fuel in a region that gets freezing temperatures, it will have been blended for that at the refinery or the fuel distributor.
It doesn't hurt to add an anti-gel additive if you are at all unsure of the fuel you have in the tank, but you will need to run the engine awhile to get the additive-mix fuel throughtout the fuel system. It doesn't help if the tank is liquid but the fuel in the filters or the line to the injector pump is like wax. You can't just dump a bottle in the tank a few hours before the big freeze.
There are folks here and elsewhere that swear by fuel tank additives and recommend them for every tank, for a variety of reasons. I'm not one of those.