As I understand IPTC date is a date format IPTC field that you can modify as you wish to your liking and personal needs. If you wish you can store birth dates of the photo subject if it is important for a photo archive like the portraits of the employees of your company - say editorial portraits of the authors or ID photos for accreditations. But you can also use it to store first publication or copyright registry dates for some pictures. Or other dates that might be important to your photography like production dates of historic cars you photograph or any other custom uses you can come up that can be some times greatly different than capture, file creation and file modification times.
