The set-date-of-modification-on-save feature request is already on the tracker
. It's waiting for the options screen, because the user would have to opt-in to this behavior. I don't want Sigil to automatically update the book's metadata by itself unless told to do so, that goes against the Law of Least Astonishment.
The date of creation being set automatically... I don't think that's what the date of creation is for. As I understand it, it's for the date of creation of the original work, not the epub file. So if you're editing a book published in 1948 but created in 1945, the DoC would say 1945 and DoP would say 1948.
That's just my interpretation, not necessarily the correct one.