Indeed, date/time of scanning is not the most useful for a timeline. Then again, it is the original date of the scan, which can be useful as well, for tracing purposes. So changing it to the date of the original negative might not be optimal either.
But is dt the best tool to editsuch metadata? It’s basically an image editor, so for me that means it should not touch metadata that are not supposed to be changed when editing.
I’d say for the case you mention (scanned negatives), either the acquitision program should allow adding such information, or a third program (exiftool, exiv2, …). Basically, as close as possible to the acquisition.
And again, not dt, as it should be a special case for most users, and it concerns fields that are not supposed to be edited. Using an external program helps reinforcing that notion of “don’t do this unless you know why you should”.