gnus-article-time-format

gnus-article-time-format is a variable defined in `gnus-art.el'.
Its value is

"%a, %d %b %Y %T %Z"

Documentation:
Format for display of Date headers in article bodies.
See `format-time-string' for the possible values.

The variable can also be function, which should return a complete Date
header. The function is called with one argument, the time, which can
be fed to `format-time-string'.

You can customize this variable.