public Builder setStartTime(com.google.protobuf.Timestamp value) {
      if (startTimeBuilder_ == null) {
        if (value == null) {
          throw new NullPointerException();
        }
        startTime_ = value;
      } else {
        startTimeBuilder_.setMessage(value);
      }
      bitField0_ |= 0x00000002;
      onChanged();
      return this;
    }
<pre>
Output only. Marks the beginning of execution.
</pre>

<code>.google.protobuf.Timestamp start_time = 2 [(.google.api.field_behavior) = OUTPUT_ONLY];
</code>
java-workflow-executions/proto-google-cloud-workflow-executions-v1/src/main/java/com/google/cloud/workflows/executions/v1/Execution.java