Function forest_filecoin::cli::subcommands::chain_cmd::tipset_by_epoch_or_offset

async fn tipset_by_epoch_or_offset(
    client: &Client,
    epoch_or_offset: i64,
) -> Result<Tipset, ClientError>

If epoch_or_offset is negative, get the tipset that many blocks before the current head. Else treat epoch_or_offset as an epoch, and get that tipset.