Why does my milk bottle lid have two dates I'm living in Switzerland at the moment. 
I've just come back from a short holiday and realise I've left some milk in the fridge.

The top oddly has two dates. One marked a which was yesterday and one marked b which isn't for another week.

Knowing that swiss milk tends to be nuclear and last a while I tried the milk and...it does seem a bit off. The water and the solid have seperated slightly in some places. It doesn't smell but it does taste odd.

I think it's fair to say I shouldn't use it. I should do without milk for a day.

However the question remains: what are these two dates?  

I guessed best before and use before but that seems wrong.
I haven't been across the border in a while, but I just double-checked via image search.

Most dairy products (and other foods with a short shelf life) in Switzerland have or had two dates, a "sell-by" and a "best before" or "consume by" date. 

Often, there will be a descriptive text, sometimes even in German, French and Italian, but there are brands that omit the text, probably because the average Swiss consumer knows how to interpret the double dates.

An example of a non-dairy product is this:


Source

Interestingly, there was a debate about the double date, centered around the confusion of consumers and their impact on food waste. Some supermarket chains like Coop have removed the "sell by" date since 2016, Migros insists on keeping it. (Source, in German)