Let's tease apart the issues, because they're messy:
The ABC "Date" field in the LR slideshow shows the capture time of the photo, as recorded in the LR catalog.
When you first import a photo into a catalog, LR reads the catalog capture time from the fields EXIF:DateTimeOriginal, XMP:DateTimeOriginal, or XMP:DateCreated. (According to the standard, it should also get it from IPTC:DateCreated, but it doesn't, which is a bug.)
You change the catalog capture time using the Metadata > Edit Capture Time command. Using this command will change the metadata fields EXIF:DateTimeOriginal, XMP:DateCreated, and IPTC:DateCreated. Changing the IPTC Date Created field in LR's Metadata panel does not change the catalog capture time.
So, if you have a lot of photos in your catalog with just the IPTC:DateCreated field set and not EXIF:DateTimeOriginal or XMP:DateCreated set, and you want the LR catalog capture time to be set to the value of IPTC:DateCreated so you can show the capture time in slideshows, you have a couple of options:
- Use the Metadata > Edit Capture Time to change each photo individually.
- Write all the metadata from the catalog back to the photos using Metadata > Save Metadata To File, then use the free Exiftool to copy IPTC:DateCreated to XMP:DateCreated. (Exiftool has a fairly steep learning curve unless you're comfortable with command-line tools.) Then reread all of the photos' metadata back into the catalog using Metadata > Read Metadata From File. (I don't know of a plugin that will do this -- the Search Replace Transfer plugin can't do it.)