To get this service for free, just print your document to PDF, grab the free Adobe Reader, go to settings, signatures, click "Configure timestamping", then add a TSA server like:
http://timestamp.comodoca.com/rfc3161
or
or one of many others that are available. Then click "Work with certificates" in the sidebar and then Timestamp document.
As long as you can convince a judge that these are neutral third parties (very easy to do and there is much precedent) this should be satisfactory.
Or you could timestamp into the block chain, but then be prepared to spend a lot of time explaining to a confused judge why it's valid.