I have a similar issue.
I want to get all the IPTC tags plus creation date and time from a source file to a destination file with transfer from EXIF:CreateDate to IPTC:DateCreated and IPTC:TimeCreated. Im must get date and time from EXIF because my soft (bibblepro) doesn't set them in IPTC.
The best I have managed to do is :
exiftool -tagsfromfile src.tif -IPTC:All '-CreateDate>DateCreated' dst.tif exiftool -tagsfromfile src.tif -d "%H:%M:%S" '-CreateDate>TimeCreated' dst.tif
The date format "%H:%M:%S" is mandatory to set the time corectly (otherwise exiftool try to set the time from the year and month which is obviously wrong).
Maybe exiftool could detect the time in a "datetime" value and use it to set a time only tag, or be abble to do a one shot '-CreateDate>DateCreated TimeCreated'.