'Show next open day' error if sunday is not first day of the week

"Show next open day" only works if sunday is the first day of the week. The setting of the field formatter is not taken into account:

<?php
// Are we currently open? If not, when is the next time?
// Remember: empty days are not in $items; they are present in $days.
if ($day <= $today) {
  // Initialize to first day of (next) week, in case we're closed
  // the rest of the week.
  if ($today == 0) {
    // Not for sundays, since this is already first day of the week.
  }
  elseif ($next === NULL) {
    $next = $day;
  }
}
?>