start date - это, по сути, дата применения политики.
A user receives an item in the Inbox folder on January 26, 2011.The Inbox folder does not have a retention tag applied, and the retention policy does not contain a default policy tag. The user soft-deletes the item on March 27, 2011. The item is then moved to the Deleted Items folder, which has a retention tag configured to delete items in 30 days. The Managed Folder Assistant processes the mailbox on March 27, 2011 and determines that the item does not have a start date. It stamps the current date as the start date, and April 27, 2011 as the expiration date.
The item will be deleted on April 27, 2011, which is 30 days after it was deleted or moved to the Deleted Items folder.