Zotero can recognize almost any date format. It puts "y m d" next to the date to show that it detected a year, month, and day. If the date does not contain a day (e.g., August 2009), then you'll see "y m" there, and if the date is only a year, then you'll see "y" there. This is just feedback about what was detected. If you mouse over the date, you'll notice that Zotero displays the internal representation in yyyy-mm-dd format (assuming it's detected correctly).
Well, it's a shorthand that many people have learned to use, and there's nothing certainly wrong with it. You can always hover over the date to see the exact date that was recognized.
I assumed that dates needed to be in y m d order, since that's what this shows!
Removing the y m d would be more helpful, if it can't be shown in the order recognized.