[1] なんか、 Opera 9.61 って valueAsDate 変更しても value に反映されないみたいなんですが。。。
[2] (X)HTML5 Tracking ( 版) <http://html5.org/tools/web-apps-tracker?from=3749&to=3750>
[3] Web Forms 2.0 ( 版) <http://www.whatwg.org/specs/web-forms/current-work/#valueasdate>
[4] Remove the valueAsDate branch from valueAsNumber (annevk著, ) <https://github.com/whatwg/html/commit/4ac4e10c8d5cc7fad67190365e4f823b64ad085d>