http://3cbzkrvakrpetjjppdwzbzqrlkmzatjs7jbyazap5gwutj32gcltjpqd.onion
First up, let's look at the PDF metadata. pdftk has a dump_data command that dumps the metadata in the document, including the creation date and the modification date. My file had both set to identical timestamps in June, both listed in UTC, corresponding to the time I'd signed the document.