That it's not obvious. It's not a standard format and a lot of people will be slightly confused. It's understandable if you think about it but still not something everyone will get at first glance. Also the fact that it's localised to US only but both digits could be a day or a month will be even more confusing.

Just adding a .18 or even better making it 2.09.18 would have made things easier and I can't believe that it would have hurt the page design that hard.