When you enable a retention tag, you must specify a retention age for the tag. This age indicates the number of days to retain a message after it arrives in the user's mailbox.
The retention age for non-recurring items (such as email messages) is calculated differently than items that have an end date or recurring items (such as meetings and tasks). To learn how retention age is calculated for different types of items, see How retention age is calculated in Exchange Server.
Там же в этой статье есть сразу ссылка как считается retention age)