Before iOS7 beta was announced, I developed this flat date picker for Xing app, which we applied for Evernote DevCup contest (You can vote us from this url: https://www.hackerleague.org/hackathons/evernote-devcup-2013/hacks/xing ). Althought it is not a real flat design, it includes gradient to provide the depth and illusion of scrolling. But if you look at iOS7 new date picker design, it is the same concept, it still provides some sort of reflection to reality. So, before iOS7 was officially released, you may want to try this control, and I think it is a good match to FlatUIKit.
And also watch the sample app video here: http://youtu.be/ROgQzdq8CXg
SSFlatDatePicker is in cocoapods now, you can find it and install it throught pods command.