Never mind whether one uses ordinal or cardinal numbers in dates. To my ears, "September 11th" sounds daft, implying as it does that there have been ten previous Septembers (cf. Henry VIII, Pope Benedict XVI). "11th September" is clearly an abbreviated form of "11th day of September" and thus makes more sense.

What I want to comment on is not the substitution of "that" for "who/whom" but the substitution of "that" for "which" in defining relative clauses, now distressingly common. I don't mind that usage changes over time, except when changes are being driven by ignorance, which is usually the case nowadays. This link expands on these points. http://dennishodgson.blogspot.com/2010/04/relatively-incorrect.html