Class ObjectMetadataPatchBuilder (2.15.1)

  namespace gcs = ::google::cloud::storage;
  using ::google::cloud::StatusOr;
  [](gcs::Client client, std::string const& bucket_name,
     std::string const& object_name) {
    auto original = client.GetObjectMetadata(bucket_name, object_name);
    if (!original) throw std::move(original).status();

    auto const tp = std::chrono::system_clock::now() - std::chrono::hours(48);
    auto updated =
        client.PatchObject(bucket_name, object_name,
                           gcs::ObjectMetadataPatchBuilder{}.SetCustomTime(tp));
    if (!updated) throw std::move(updated).status();

    std::cout << "The custom time for object " << updated->name()
              << " in bucket " << updated->bucket() << " was successfully set. "
              << "Full object details: " << *updated << "\n";
  }