Why a job would be removed if it doen't have next execution time?

[eureka176]

I add a 'date' job like this, and i was failed to get status because it is removed already, can anyone teach me how to get this job's status, thank a lot!
def retrieve_data():
while True:
print('The job is running at %s' % (datetime.datetime.now().strftime('%Y-%m-%d %H:%M:%S')))
time.sleep(3)

i just wondering why the job will be removed:
# Update the job if it has a next execution time.
# Otherwise remove it from the job store.
job_next_run = job.trigger.get_next_fire_time(run_times[-1], now)
if job_next_run:
job._modify(next_run_time=job_next_run)
jobstore.update_job(job)
else:
self.remove_job(job.id, jobstore_alias)