Trustless time from blockchain, get the current time without trusting any root

I think the general idea of the using blockchain in long-lived IoT devices seems pretty dangerous. In the last 3 years, Ethereum had hard-forked twice already. A hypothetical IoT device made 3 years ago would not work again, if it were relying on the blockchain.

At the same time, TLS 1.1 is 13 years old, and is still supported by all the modern software libraries. Make a TLS certificate with 25 years lifetime, hardcode its fingerprint into your app, and you'd have nice, simple and much more reliable timesync service.

This will not be decentralized, but it does not matter -- your IoT device has a trusted server anyway, this is the server that gives it the firmware updates. Since you fully trust this server, you might as well use if for time and make your life much easier.