A possible solution would be to transform the day
field of the date component to a hidden
field instead of the select
field type. That can be achieved by adding a #process
callback for that field and altering the data.
function YOURMODULE_form_alter(&$form, &$form_state, $form_id)
{
// Your logic here depending which form to alter
// ...
// Add #process for the component with key name 'date'
$form['submitted']['date']['#process'][] = 'YOURMODULE_process_date';
}
function YOURMODULE_process_date(&$element)
{
// change type to hidden
$element['day']['#type'] = 'hidden';
// set value to first day of the month
$element['day']['#value'] = '1';
return $element;
}