There exists a dedicated timestamping RFC: RFC3161. And yes, there is a vast number of publicly available (for a fee) service providers that offer this as a ser

There exists a dedicated timestamping RFC: RFC3161. And yes, there is a vast number of publicly available (for a fee) service providers that offer this as a service. Even before blockchain, these were in use. In Germany, there was a law that would require you to store you bookkeeping transactions only with those timestamps applied to avoid retroactive tampering. I.e. if you had received an Invoice as a PDF, you were required to timestamp it in a "secure, tamper proof manner" to be legally compliant. I personally used the free RFC3161 service provided and run for free by DFN ("Deutsches ForschungsNetzwerk", an organization of universities and higher education institutions). The law was abandoned, and it is okay now to store digital invoices without timestamps. If I would have been audited, I am not sure that those timestamps would qualify to met the requirements by law, but I took my chances. To this day there are still various providers of RFC3161 timestamps, and interfacing is rather easy (you send an HTTP post request by cURL and receive a signed fingerprint by the trusted party).