PS: I think the proper format for web apps is that which is simplest to use In some cases a timestamp, in other cases YYYY-MM-DD or anything else that is simple to work with and that suits the clients side needs...
Input fields like <input type="date"/>, <input type="datetime"/>, tags <time datetime="2013-02-01"></time> expect YYYY-MM-DDTHH:MM:SS.SSS+ZHZM format, that's why I think it should be default in skylark.