public com.google.type.Date getPublicationDate() {
      if (publicationDateBuilder_ == null) {
        return publicationDate_ == null
            ? com.google.type.Date.getDefaultInstance()
            : publicationDate_;
      } else {
        return publicationDateBuilder_.getMessage();
      }
    }
<pre>
Output only. Publication date of the attribution.
</pre>

<code>.google.type.Date publication_date = 6 [(.google.api.field_behavior) = OUTPUT_ONLY];
</code>

@return The publicationDate.
java-vertexai/proto-google-cloud-vertexai-v1/src/main/java/com/google/cloud/vertexai/api/Citation.java