Silverstripe setValue in updateCMSFields doesn't work

public function updateCMSFields(\FieldList $fields) {
        parent::updateCMSFields($fields);

        $field = new DateField('FromDate');
        $field->setConfig('showcalendar', true);
        $field->setConfig('dateformat', 'dd/MM/YYYY');
        $field->setTitle(_t("BlogEntry.TODT", "Date"));
        if ($this->owner->Created == $this->owner->LastEdited) {
            // In caso di nuovo post, setto il campo FromDate ad oggi
            $now = Zend_Date::now();
            Debug::log('New FromDate: '.$now->toString('d/M/Y'));
            $field->setValue($now->toString('d/M/Y'));
        }
        $fields->addFieldToTab('Root.PublishedPeriod', $field);

        $field = new DateField('ToDate');
        $field->setConfig('showcalendar', true);
        $field->setConfig('dateformat', 'dd/MM/YYYY');
        $field->setTitle(_t("BlogEntry.TODT", "Date"));
        $fields->addFieldToTab('Root.PublishedPeriod', $field);
    }