I had the same problem with the workflow. It stopped working once I have added an almost empty Bibtex entry in Bibdesk (just attached the pdf without filling in the fields). Since I do that quite often I thought it would be a good idea to make the workflow a bit more failsafe by giving the date value a standard value of 0 if none exists (ccl_bplist.py, line 123--127):
elif type_byte & 0xFF == 0x33: # Date 0011 0011
date_bytes = f.read(8)
date_value = __decode_float(date_bytes)
if date_value < 0: date_value = 0
return datetime.datetime(2001,1,1) + datetime.timedelta(seconds = date_value)