The fun of competing standards. In regards to the XMP versions of EXIF:DateTimeOriginal, the IPTC standard uses XMP-photoshop:DateCreated while the EXIF for XMP standard uses XMP-exif:DateTimeOriginal.
The xmp2exif.args copies XMP-exif:DateTimeOriginal as part of XMP-exif:all, but XMP-photoshop:DateCreated copies into Composite:SubSecDateTimeOriginal. This means that the XMP-exif:DateTimeOriginal will not fill SubSecTimeOriginal/OffsetTimeOriginal.
I would suggest that this be added.
-Composite:SubSecDateTimeOriginal < XMP-exif:DateTimeOriginal
Just a minor change and not a high priority.
Good point, thanks. I'll change this in the next release.
- Phil