Hi

I did some more testing and error handling on my code I posted in  
<archive.pu7pc20r.fsf@hschmi22.userfqdn.rz-online.de>.

It should work without problems. What it does is the following:
Add a new item (%q) to gnus-summary-line-format-alist which calls
gnus-user-date(date-header).

gnus-user-date formats the date accordingly to 
gnus-user-date-day
gnus-user-date-week
gnus-user-date-old

That means that if the message is of today you get e.g. 10:24
if not older than 7 days you Mon 12:35
and if it's older you Okt 16

gnus-user-* can hold any valid value for format-time-string.

here are the patches: