Possibly the date that a message was sent can be shown some where.  I have notices that there is always a 10 messages that appear on my front page. When I read a message,  a new one appears on the list. If there is a message that relates to a notification/ invitation to a group match, I do not know the date, unless I follow the the link back to determine if the match has already started.

When you look at the messages in your inbox, on the right side column under the senders name it shows the date the message was sent to you.

Also, if you use the 'Search Messages' box on the right side, and enter the name of the message sender, all messages received from that specific person will show for you in one list.

Gotcha.  That explains it.