I think you're mistaking the date label that appears beside the 'Seen' checkbox' as the date field. It is a little confusing. In the standard skin, the actual field is down with the other dates. If no date has been set, it will only appear in edit mode. If today's date has been set by checking the box, the date still won't appear in the date field until after a refresh.
Note the [Seen date] can also be set from the list context menu (i.e., right-click on the title in the list). You'll understand what's happening better if you try that, refresh (F5), then note the resulting changes to the flag and the actual date field.