A timestamping authority (digital notary)
If you search on "timestamp", "timestamping", and "notary" on Crypto.SE and Security.SE, you'll find lots of references.  I've collected a number of timestamping services that were mentioned in one of those places; this should provide a number of companies and online services you can check out:


http://www.proofofexistence.com/
https://www.btproof.com/
https://www.universign.eu/en/
http://www.guardtime.com/
http://securo.it/
http://www.cryptostamp.net/
http://www.itconsult.co.uk/stamper.htm
https://www.globalsign.com/timestamp-service/


I cannot vouch for or endorse any of those.

See, for instance, the following questions on this site, and the following other resources:


https://security.stackexchange.com/q/19184/971
https://security.stackexchange.com/q/43995/971
How are timestamps verified?
Signature and Timestamp for Long Term Document Archival Question
Timestamping services authorities, utilizing Linked, hybrid and other than PKI schemes
https://security.stackexchange.com/q/24458/971
https://en.wikipedia.org/wiki/Trusted_timestamping
http://tools.ietf.org/html/rfc3161
https://security.stackexchange.com/q/32493/971
https://security.stackexchange.com/q/47289/971


Happy reading!