form api - pass timestamp with date type form element

Beginning to toy with impressive form api in 4.7.

I found I needed to do some validation against a date element. I used the nice automatic item, i.e.:

$form['details']['date'] = array(
  '#type' => 'date',
  '#title' => t('Start date')
);

Then I needed to do some validation. First I wanted to check if the user had set any date at all, but since the date #value automatically defaults to today's date there's no immediately obvious way to distinguish between "not set" and "set for today". Also, no way to doublecheck that a date in the past hasn't been chosen. Other scenarios too..

Anyway, to cut to the chase - I hacked form.inc to pass the timestamp of the default date along with the other info. In the form.inc function expand_date I replaced the first few lines at 788:

// Default to current date
  if (!isset($element['#value'])) {
    $element['#value'] = array('day' => format_date(time(), 'custom', 'j'),
    'month' => format_date(time(), 'custom', 'n'),
    'year' => format_date(time(), 'custom', 'Y'));
  }

with the following. Note that I've moved generation of date element values outside the conditional because, in the event of a form_error on another form element, we would lose the "today" info if user has already set a date.

  $this_day = format_date(time(), 'custom', 'j');
  $this_month = format_date(time(), 'custom', 'n');
  $this_year = format_date(time(), 'custom', 'Y');
  //question: mktime or gmmktime?
  $today = gmmktime(0, 0, 0, $this_month, $this_day, $this_year);
  $today = format_date($today, 'custom', 'U');
  if (!isset($element['#value'])) {
    $element['#value'] = array('day' => $this_day,
      'month' => $this_month,
      'year' => $this_year);
  }

Then, at the end of the function, I added the following to generate the extra form item:

$parents[] = 'timestamp';
$element['timestamp'] = array(
   '#type' => 'value',
  '#value' => $today
);

Finally, in my own module's _validate (or _submit) function, I convert the submitted date thusly:

$formday = gmmktime(0, 0, 0, $edit['date']['month'], $edit['date']['day'], $edit['date']['year']);
$formday = format_date($formday, 'custom', 'U');

Which gives a timestamp of same format, with timezone synched to the original timestamp, etc.

Comments? Improvements? Worth a patch?