If you want to copy the date/time value of a specific metadata field into another metadata field, the Metadata Template is an easy way to do so. Just make sure you use the date/time format that ExifTool requires.
Example: Copy the Date/Time in Exif\CreateDate into XMP\CreateDate:
Tag:
XMP::xmp\CreateDate
Fill the tag from this data:
{File.MD.Exif::Main\36868\CreateDate\0|format:YYYY:MM:DD hh:mm:ss}