Depends. Most companies that deliver off road diesel will treat it for Winter use. How much treatment they use and when they start treating it will vary from one place to another depending on what part of the country they are in and how much they want to spend on treating their fuel. Near me they start treating in October.
Personally I treat all of my own fuel and probably over treat it, partly because I don't trust the fuel companies to get it right. Plus there is some dilution to be expected with Winter fuel being dumped into summer fuel. Usually no big deal in October, but I like to make sure. Standing in freezing weather next to a machine that wont start with your fuel covered hands on cold metal is not my idea of a good time.