UK versus US expiration dates on diet sodas I took a trip to the UK last week, and as an avid Coke Zero drinker, was surprised to find that my Coke Zero supposedly expired in March 2016 in most places I went.

Surprised, because in the US, diet sodas are typically dated 3-4 months from production, and actually do taste noticeably less sweet at that point (to the point that some brands of soda, including Coke Zero, are undrinkable for me if they're at the expiration date).  March 2016, even assuming they were produced next door and stocked that day, was almost 6 months out, and considering about 75% of the sodas I found were that date (I did see some Jan and Dec, but almost all Mar) it sounds like they're dated at least six months out.  But even the Dec dated sodas tasted fine (Which, if they were 3 months old, would start tasting off to me normally in the US).

Why is that? Does the UK have different regulations, or are there different formulas that explain it, or different storage, or ... something else?  The base sweeteners were the same (Aspartame and Ace-K), though of course it's impossible to tell %s in the US so who knows if they're the same; and I think it all tasted differently there than in the US, so it's hard to say if there's more/less in there.
First of all, sweeteners like Aspartame lose their sweetening property over time, as you have noticed yourself. This is very strongly tied to the pH level they are in.

US and UK/EU coke (or any country) is different, but you usually only notice the difference if you have them available for direct comparison. Coca Cola issues licenses and the plant managers produce the coke, but they do have a certain bandwidth they can operate in - usually to cater to local issues, like component availability, price, water quality. You can even taste differences in different batches of the same production plant, if you have them available for direct comparison and if you have a trained taste.

So, the deviation of the pH level in the different products results in different best before dates. The US coke pH level deviates more from the optimal level for Aspartame than the UK coke, so the US coke has a shorter best before date.

Whether the different pH level is on purpose or a necessity: no idea.