Guardtime is set of tooling around a FIPS 140 timestamping appliance. Systems on the network submit hashes of their logs to the time stamping. Assuming the appliance remains secure, you can go back in the event of compromise and prove that your logs are unaltered up to a certain point. It's amazing that they seem to have built a $50 -$100 million dollar business here.
Makes me wonder what mathematics would prove this, or if it's assumed to be out of scope, and if so, what else is out of scope too.