private function determineDateForPrefix($startDate, $endDate)
    {
        // The default date value should look like "*/*/*" after joining
        $dateParts = array_fill_keys(['Y', 'm', 'd'], self::PREFIX_WILDCARD);

        // Narrow down the date by replacing the WILDCARDs with values if they
        // are the same for the start and end date.
        if ($startDate && $endDate) {
            foreach ($dateParts as $key => &$value) {
                $candidateValue = date($key, $startDate);
                if ($candidateValue === date($key, $endDate)) {
                    $value = $candidateValue;
                } else {
                    break;
                }
            }
        }

        return join('/', $dateParts);
    }