vs date URLs

How do this list archives employ this type of URL:
http://www.spinics.net/lists/mhonarc/thrd4.html

where the url has thrd4 versus a date based URL like

http://www.spinics.net/lists/mhonarc/2007-05

?

J

---------------------------------------------------------------------
To sign-off this list, send email to majordomo(_at_)mhonarc(_dot_)org with the
message text UNSUBSCRIBE MHARC-USERS