OpenTimestamps is a tool that enables users to create timestamps and to verify them. Timestamps prove some data existed prior to a point in time. Users can create timestamps in permissionless blockchain systems such as Bitcoin and Ethereum but it requires paying the network fee. OpenTimestamps solution to this is using a calendar server which is free to set-up. Users can verify timestamps on the OpenTimestamps website by dragging and dropping a file.
Users can also sign up to a mailing list to keep up-to-date with announcements. Companies currently using OpenTimestamps include Eternity Wall, Signatura, and Cerved. For example, Eternity Wall's service enables users to write eternal, uncensorable and timestamped messages. The timestamped property is where the use case for OpenTimestamps come in.