I just noticed with the current 2.0.40 that the imagedate field in the database is not set correct for some pictures. (As I am importing all my pictures from Gallery3 to NGG I noticed that it works for some galleries, but not for others. So if it works, it works for all images, if not, for none.)
When I view the meta data in the backend of NGG, everything looks fine. I see all kinds of information, also the correct image date and time. But not in the gallery overview, here I see the import date/time, which means today, NOW().
Also I cannot sort with date/time because of this.