The best before date on milk starts from the date of packing and varies from processor and product. Why? There are different ways to pasteurize milk (varying duration and temperature) and the amount of fat in the product will affect it as well.
The date labelling guidelines are set by the Canadian Food Inspection Agency, regarding durable life, best before dates, expiry dates, and “use by” dates, and even states how that date must appear on a package. You can learn more about his on their website.