CRM_Contribute_Form_Task_PDFLetterCommon contains problem UPDATE for the receipt and thank you times

civicrm_api("Contribution", "update", array('version' => '3', 'id' => $contributionId, 'receipt_date' => $nowDate));

should be

civicrm_api("Contribution", "create", array('version' => '3', 'id' => $contributionId, 'receipt_date' => $nowDate));

same for the thankyou_date

Refer to http://forum.civicrm.org/index.php/topic,26000.15.html

I had to track this down when the user reported breakage. Call to Contribution Update produces strange results, eg the fee amount is invalid, then the net_amount is invalid. Set those values, and it's fatal.

"create" seem to work fine.