Scott,
that date is kind of the standard beginning of time date n the IT world on all Unix originated operating systems, including OS X.
It shows, that most probably the PDF is in a format that ArchiCAD could not open. As a result it could not read the correct date neither the content. Do you know the source application where the PDF was saved?
As a quick workaround open the PDF in e.g. Preview and save it again. Place this new PDF in ArchiCAD and let me know the results.
Marton
Marton Kiss
Chief Product Officer
GRAPHISOFT