public Builder clearPublicationDate() {
      bitField0_ = (bitField0_ & ~0x00000020);
      publicationDate_ = null;
      if (publicationDateBuilder_ != null) {
        publicationDateBuilder_.dispose();
        publicationDateBuilder_ = null;
      }
      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