/**
* Restrict on a date field.
* 'from' or 'to' may be empty.
* @param string $field
* @param DATE_TIME $from
* @param DATE_TIME $to
*/
public function restrict_date($field, $from, $to)
{
if ($from && $from->is_valid()) {
$this->restrict("{$field} >= '" . $from->as_ISO() . "'");
}
if ($to && $to->is_valid()) {
$this->restrict("{$field} <= '" . $to->as_ISO() . "'");
}
}
