Edward,
It depends on your IPTC/XMP Preferences in Photo Mechanic. If you've instructed PM to read XMP before IPTC then it is the DateCreated field as described above. Otherwise it will come out of a binary IPTC record field, and if that doesn't exist, it will use the EXIF Capture Date/Time tag.
What is a binary IPTC record field and how would it show up? I know this is not your area, but would it show up in ExifTool?
Binary refers to the way the data is encoded. If you look at an IPTC record in a file editor, much of the structure is just numbers, but the strings of text are human readable.
ExifTool should be able to present the data for you.
-Kirk