The W3 validator says that placeholder attributes are not valid for date entries. Checking this HTML:
<!doctype html>
<title>date placeholder test</title>
<input type="date" placeholder="enter a date">
Gives an error: " Attribute placeholder not allowed on element input at this point." ... and says that you can use the attribute " placeholder when type is text, search, url, tel, e-mail, password, or number".
In addition, Chrome does not show a placeholder for entering the date, although it believes that this attribute exists in JS (this is about how Modernizr checks the attributes):
var test = document.createElement(element);
test.type = 'date';
alert('placeholder' in test);