I want users of level 1 (and above) to be able to change the timestamp on their posts.
I found this (too old?) thing: http://wordpress.org/support/topic/16789
But that doesn't seem to do it all the way... (the function touch_time in admin-functions.php had changed since that support topic was posted. So I did not change this function, since I think that isn't needed...)
Users of level 1 can now indeed see the "edit timestamp" option and indeed can change it.
BUT (for all levels of users) the timestamp can only be set when writing the post FOR THE FIRST TIME. When I go back to a concept I can not change the timestamp anymore... The change timestamp options still shows up, but changing the timestamp and clicking the save-button doesn't update the timestamp anymore...