gnus-user-date-format-alist

gnus-user-date-format-alist is a variable defined in `gnus-sum.el'.
Its value is shown below.

Documentation:
Specifies date format depending on age of article.
This is an alist of items (AGE . FORMAT). AGE can be a number (of
seconds) or a Lisp expression evaluating to a number. When the age of
the article is less than this number, then use `format-time-string'
with the corresponding FORMAT for displaying the date of the article.
If AGE is not a number or a Lisp expression evaluating to a
non-number, then the corresponding FORMAT is used as a default value.

Note that the list is processed from the beginning, so it should be
sorted by ascending AGE. Also note that items following the first
non-number AGE will be ignored.

You can use the functions `gnus-seconds-today', `gnus-seconds-month'
and `gnus-seconds-year' in the AGE spec. They return the number of
seconds passed since the start of today, of this month, of this year,
respectively.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.

Value:

(((gnus-seconds-today)
  . "Today, %H:%M")
 ((+ 86400
     (gnus-seconds-today))
  . "Yesterday, %H:%M")
 (604800 . "%A %H:%M")
 ((gnus-seconds-month)
  . "%A %d")
 ((gnus-seconds-year)
  . "%B %d")
 (t . "%b %d %Y"))