/**
  * Toggles the checkout state of the given allocation and position in allocationCells.
  * $allocationId : id of allocation
  * $posn : position within allocationCells to toggle (multiple checkout dates may be possible for any given allocation)
  */
 function toggleCheckoutOnBookingDate($allocationId, $posn)
 {
     // find out the date that was toggled by adding the offset to minDate
     $checkoutDate = clone $this->showMinDate;
     $checkoutDate->add(new DateInterval('P' . $posn . 'D'));
     AllocationDBO::toggleCheckoutOnBookingDate($allocationId, $checkoutDate);
 }