[whatwg] input[type=date] placeholder attribute and displayed date format

[Ben Darlow]

I've recently been implementing date controls for a web application to be
primarily used on mobile browsers, and discovered that the placeholder
attribute is considered forbidden for use on input[type=date] elements. I
would like to propose that this be changed.

Since the spec does not (and should not) mandate the specific
implementation of the interface to a date element, not permitting a
placeholder means that where a browser chooses to implement the control in
a similar vein to an ordinary text input (i.e. with free text entry), this
control can't follow the same pattern of label + example via placeholder.
I've mocked this up with a use case which I foresee as being relatively
common where the browser implements the control like this:
https://img.skitch.com/20120117-tmh1bu3wxcarxr94kyk147h3k2.png

Another issue with input[type=date] elements relates to how the *value* of
the date and how it is *displayed* may want to be different. In iOS 5.0,
Mobile Safari implements the input[type=date] control with a native spin
control, but as this is independent of how the control is displayed within
the webpage itself (unlike the more text input-like approach), it also
formats the date in a different format to the ISO format that
input[type=date] mandates (example:
https://img.skitch.com/20120117-grwdr7ebd56iww617ypatqqa88.png).

The ISO date format has the advantage over different local date formats in
that it is unambiguous, but it isn't necessarily the most user friendly. I
would like therefore to propose that an optional dateformat attribute be
added to the input element, to permit different formats to be used to *
display* a selected date. My initial feeling is that some flavour of the
strftime syntax implemented by several languages (e.g.
http://ruby-doc.org/core-1.9.3/Time.html#method-i-strftime) is probably the
best way of specifying the format within this attribute.

An example usage of this might thus be:

<input type="date" name="dateofbirth" dateformat="%-d %b, %Y">

This would then create a date input whose selected date would be formatted
as per the screenshot above.

Comments or observations of important points I've missed welcomed!

~B