Get the hash value associated to the data file to be timestamped.
No internet connectivity is required: the hash is calculated inside your browser without disclosing your document to third parties, preserving your privacy.
Submit the hash value to multiple OpenTimestamps calendar servers for attestation of its existence (timestamp) and save the OTS proof as local file; internet connectivity is required.
However, the proof cannot be verified immediately: it is incomplete as it takes a few hours for the timestamp to get confirmed by the Bitcoin blockchain; OpenTimestamps is not doing one transaction per timestamp: calendars do aggregate multiple documents that are later attested with a single blockchain transaction.
Load an existing OTS proof from a local file (no internet connectivity required).
Display the info included in the OTS proof, including the actual commitment operations and attestations (no internet connectivity required).
Attempt the upgrade of the OTS proof with attestations that might be available from the OpenTimestamps calendars (internet connectivity is required). Incomplete proofs require the assistance of the remote calendars used for timestamping to be verified: calendars provide the path to the Bitcoin block header. Upgrading an incomplete timestamp adds the path to the Bitcoin block header to the proof itself, making it self-contained and calendar-indipendent.
Verify the attestations included in the OTS proof (internet connectivity is required).
Timestamps can be verified independently using a local Bitcoin Core node; anyway, since a web-page cannot access the local filesystem, it relies on public blockexplorers.
Proof upgrading is attempted contacting the OpenTimestamps calendars before verification: if an upgrade is available, then it is downloaded and used for verification.