This is an anoying problem that occurs in the program member list and in the Admin | Users list (see related Idea below). But from your prior Disucssion item, I understand that you encountered this in a smart list, which for me displays the SFDC Created dates (and other dates) in the format "yyyy-mm-dd hh:mm" and sorts properly. I'd be very curious why your smart list is displaying this format. In Admin | Users & Roles, the "User since" column should sort on date