<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def change_execution_time(self, job, date_time):
 """ Change a job's execution time. """

        with self.connection.pipeline() as pipe:
            while 1:
                try:
                    pipe.watch(self.scheduled_jobs_key)
                    if pipe.zscore(self.scheduled_jobs_key, job.id) is None:
                        raise ValueError('Job not in scheduled jobs queue')
                    pipe.zadd(self.scheduled_jobs_key, {job.id: to_unix(date_time)})
                    break
                except WatchError:
                    # If job is still in the queue, retry otherwise job is already executed
                    # so we raise an error
                    if pipe.zscore(self.scheduled_jobs_key, job.id) is None:
                        raise ValueError('Job not in scheduled jobs queue')
                    continue