>I am trying to understand the DATEDIFF() function better. Do I understand correctly that the 1st parameter "dd" really has no literal meaning? That is, you can name this parameter anything and it won't change the result?

The first parameter says, that we need to get difference in days.

http://msdn.microsoft.com/en-us/library/ms189794.aspx

http://doc.ddart.net/mssql/sql70/da-db_6.htm