OpenTimestamps offers a timestamp recording service, currently making use of Bitcoin as a decentralized source of truth. It is not totally decentralized like Skynet however; the intermediary between the user and the blockchain is some kind of server. (OpenTimestamps operate their own servers free of charge [e.g. https://alice.btc.calendar.opentimestamps.org/], and they have a number of backers, so it’s not the worst point of centralization there is.) The alternative to using a server would be posting the relevant data to the blockchain directly (I think), which means having bitcoin and spending it and… you get the picture.
Here’s what I am thinking. SiaCoin uses a blockchain very similar to Bitcoin, and it has the same block time. OpenTimestamps could be configured/extended to instead post to the Sia blockchain, using a SiaCoin node. From there, portals could be configured to provide the timestamping service that OpenTimestamp nodes currently provide, without needing to operate a Bitcoin node. I would suggest using Skynet instead of the Sia blockchain, but I am pretty sure there is no trustless way to timestamp something on Skynet.
With OpenTimestamps built into Skynet portals and Sia, skapps can benefit from decentralized timestamping. The current alternatives are to either (a) rely on the good faith of the users or (b) use the aforementioned server and the Bitcoin blockchain. Adding OpenTimestamps support to Sia could also make that service more robust as a whole, to the benefit of everyone in my opinion.
I am interested in knowing if this is a feasible proposition. If it is, I think it is definitely worth pursuing.