public Builder setPublicationDate(com.google.type.Date value) {
      if (publicationDateBuilder_ == null) {
        if (value == null) {
          throw new NullPointerException();
        }
        publicationDate_ = value;
      } else {
        publicationDateBuilder_.setMessage(value);
      }
      bitField0_ |= 0x00000020;
      onChanged();
      return this;
    }
<pre>
Output only. Publication date of the attribution.
</pre>

<code>.google.type.Date publication_date = 6 [(.google.api.field_behavior) = OUTPUT_ONLY];
</code>
java-vertexai/proto-google-cloud-vertexai-v1/src/main/java/com/google/cloud/vertexai/api/Citation.java