I am shocked when I meet an American who actually knows when to use PDT as opposed to PST. They also think they're being clever asking if I want to have a meeting at X:00 GMT when they mean BST (or Irish Standard Time, which is confusingly observer in Ireland in the _summer_, which is 1 hour ahead of GMT just like BST is).

Of course, when the S can mean either "summer" or "standard" one can forgive some confusion.