spoiler
Doughnut didn’t occur to me at first, but when I found out clock was wrong it was fairly obvious doughnut would be right since it’s a bread-y product and bread is famously sold by the dozen (hence the phrase "baker’s dozen").
Inches in a foot is imperial units, which I’ve never used, but unlike most of the other imperial measures, it’s a very basic bit of trivial knowledge to know there are 12. It’s something I would expect people to know even if they don’t know how many yards in a mile or ounces in a pound.
12 jurors is not unique to the US. It’s what we use here in Australia too, and quick googling tells me so does the UK, Canada, NZ, and Ireland, 100% of the countries I tried searching. It seems to be the accepted standard in legal systems based in the system of Common Law.
Dog days I know of as "the dog days of summer", a phrase I would never actually use myself but which I believe refers to the hottest and most humid part of summer. Dog paddle I’m guessing is an American term for what I would call doggy paddle; an unskilled form of swimming with your head above water and legs down. Dog tired is an expression that, again, I wouldn’t really use myself, but it just means "very tired".