Not based on "last comment || last edited" ? ;)

¥

I knew this rang a bell

blueyed on the dev list wrote:

Item::update_excerpt autogenerates $excerpt and sets it to the object,
which gets updated in DB if changed/generated.

This leads to post_datemodified also being updated.

This causes items to get the current timestamp as "last modified", if some
web spider crawls your archives (where excerpts of old items get displayed
(and auto-generated)).

¥

Fixed in CVS (no, not because of this thread). Please check.