code/Debug/Model/Cron.php
public function getExpirationDate($currentDate)
    {
        $numberOfDays = Mage::helper('sheep_debug')->getPersistLifetime();

        return date(self::DATE_FORMAT, strtotime("-{$numberOfDays} days {$currentDate}"));
    }
public function getExpirationDate($currentDate)
    {
        $numberOfDays = Mage::helper('sheep_debug')->getPersistLifetime();

        return date(self::DATE_FORMAT, strtotime("-{$numberOfDays} days {$currentDate}"));
    }
Removes configured number of days from specified date

@param string $currentDate
@return bool|string