There is no standard for best before dates, because the concept of "consume before this date" isn't scientifically valid; a huge variety of factors can go into spoiling food and most of it is safe after that date but sometimes (if you store it wrong) it can be spoiled before the date. It's the same for chemicals. My 82% isolpropyl has a "best before" date of April 2023, but unopened, it could last well into the late 2020s before enough of the alcohol molecules break down or seep into the plastic container. If I leave it opened too long today, though, it could become 72% isopropyl as the alcohol evaporates. Even the media we use for biological testing in food function long after their "best before" dates. The important part is the storage of the materials.
A lot of places use Julian dates for the year, that's where you count the days from 1 to 365. It turns a date like "20210115" into "21015", or "01521". If you have a coder that only fits so many digits (most are 8, 12, 16 or 20) that frees up a few digits to code other information like time of manufacture, facility of manufacture (if you have multiple plants), or even more complex like a system for encoding which batches of raw materials were used or which specific pieces of equipment were used. Those numbers would all reference paperwork that tracks all of that so that when you complain that your butter is off and bring it to us, we can ask "what is the code" and you say "B2750-1 11:24" and we know that it was made at plant B on day 275 (October 16th) in 202
0 on line-1 at 11:24am and find all the paperwork to see what packaging material was used, where the milk came from, how much salt was added by whom and from which supplier, what was the HPC count, what was the yeast and mould result that batch/day/week in the product/air/equipment, etc.
Anyway this is just one of the many reasons why food costs so much.
Testing costs the small business I work for thousands a year; larger companies spend that much
daily to make sure food is safe. The big beverage manufacturers sit on massive warehouses that just have hourly samples of the products they made, sitting on a shelf, for at least 2 years (some do as much as 5) so that they can 1: reference it in the event of a complaint and 2: monitor how it breaks down over time.
Anyway, to loop around to the first post: I write the date I either bought or opened a product on most of my food.
Best before dates refer only to an unopened product. If you've
opened that butter, it's been
exposed, and the oxygen in the atmosphere around it is going to start doing its thing. A jar of spaghetti sauce is good for two years, but once you've opened it and even a bit of oxygen has gotten in, it starts breaking down and the bacteria and mould spores (which are ever present and cannot be killed) will wake up and start eating it for you.