date sent --- This is when the author of the message CREATED the message. If a message is created on 1 Jan 2008, then left in the drafts mailbox and not sent until 8 April 2009, then the date sent is 1 Jan 2008. (This is something Apple can and should change.)
date received --- This is when the message arrived at the sender's ISP. Usually only a few seconds or minutes after the date sent. It has nothing to do with when the message arrived in your computer. (This should also be changed. It should probably be the date/time when the message became available to the recipient, but that may be hard to do.)