public Builder mergePublicationDate(com.google.type.Date value) {
      if (publicationDateBuilder_ == null) {
        if (((bitField0_ & 0x00000020) != 0)
            && publicationDate_ != null
            && publicationDate_ != com.google.type.Date.getDefaultInstance()) {
          getPublicationDateBuilder().mergeFrom(value);
        } else {
          publicationDate_ = value;
        }
      } else {
        publicationDateBuilder_.mergeFrom(value);
      }
      if (publicationDate_ != null) {
        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