Form select option results in "Illegal Choice has been detected"

Question

I have a form where a user picks a date for their submission(s) to appear on the website. I am populating a 'date' field like so:

function mymodule_get_date_options($type = 'start') {
  $options = array();

  $start_week = time();

  if (date('w', $start_week) == 3) {
    $start_week = strtotime('+1 week', $start_week);
  }

  while (date('w', $start_week) != 3) {
    $start_week = strtotime('+1 day', $start_week);
  }

  if ($type == 'end') {
    $start_week = strtotime('+1 week', $start_week);
  }

  for ($x = 0; $x < 24; $x++) {
    $options[date('U', $start_week)] = date('M d, Y', $start_week);
    $start_week = strtotime('+1 week', $start_week);
  }

  return $options;
}

And the element:

$form['start_date'] = array(
    '#title' => t('Start Date'),
    '#type' => 'select',
    '#options' => mymodule_get_date_options('start'),
    '#default_value' => $mymodule_form_object->start_date,
  );

The generated HTML:

// a chunk of the option elements
<option value="1313615248">Aug 17, 2011</option>
<option value="1314220048">Aug 24, 2011</option>
<option value="1314824848">Aug 31, 2011</option>
<option value="1315429648">Sep 07, 2011</option>
<option value="1316034448">Sep 14, 2011</option>
<option value="1316639248">Sep 21, 2011</option>
<option value="1317244048">Sep 28, 2011</option>
<option value="1317848848">Oct 05, 2011</option>
<option value="1318453648">Oct 12, 2011</option>

So, everything looks okay, except when the form is submitted it says 'An Illegal Choice has been detected" with the date selects highlighted.

Answer 1

That's because the form gets generated both upon output and submission, and you're using form values that are time-dependent. When you first output the form it generates options like this:

1313615248
1314220048
1314824848
1315429648

And when a user submits the form, it will generate new values, with a few second difference:

1313615268
1314220068
1314824868
1315429668
(+20 second difference)

So none of the previous options are present here. Drupal's validator by default checks if the submitted option is among generated options.