A bogus date (Wed 31Dec 1969 04:59:59 PM MST) appears in the properties of Print-to-PDF files produced by PureBrowser, Thunderbird, and LibreOffice. This might happen in all applications exporting to PDF, but so far this is all I've tested and made screenshots of.
In PureBrowser the bogus date is both Created and Modified. In Thunderbird and LibreOffice only the Modifed date is bogus. In PureBrowser cairo is listed as the PDF Producer and Creator. In Thunderbird cairo is listed as the PDF Producer, and the Creator is set to "none". Running the PDF file through GhostScript will fix both dates.
Print-to-PDF from PureBrowser:
Print-to-PDF from PureBrowser, then run through gs to reduce image size. Dates are fixed.
Print-to-PDF from Thunderbird:
Print-to-PDF from LibreOffice: