private com.google.protobuf.SingleFieldBuilderV3<
                com.google.protobuf.Timestamp,
                com.google.protobuf.Timestamp.Builder,
                com.google.protobuf.TimestampOrBuilder>
            getStartTimeFieldBuilder() {
          if (startTimeBuilder_ == null) {
            startTimeBuilder_ =
                new com.google.protobuf.SingleFieldBuilderV3<
                    com.google.protobuf.Timestamp,
                    com.google.protobuf.Timestamp.Builder,
                    com.google.protobuf.TimestampOrBuilder>(
                    getStartTime(), getParentForChildren(), isClean());
            startTime_ = null;
          }
          return startTimeBuilder_;
        }
<pre>
Optional. The starting timestamp for messages to import.
</pre>

<code>
.google.protobuf.Timestamp start_time = 2 [(.google.api.field_behavior) = OPTIONAL];
</code>
java-vertexai/proto-google-cloud-vertexai-v1/src/main/java/com/google/cloud/vertexai/api/SlackSource.java