presto-main/src/main/java/io/prestosql/operator/PipelineContext.java/PipelineContext/start
class PipelineContext:    public void start()
    {
        DateTime now = DateTime.now();
        executionStartTime.compareAndSet(null, now);
        // always update last execution start time
        lastExecutionStartTime.set(now);

        taskContext.start();
    }