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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _local_to_utc(self, timestamp, local_zone="America/Los_Angeles"):
 """ Convert local timestamp to UTC. Parameters timestamp : pd.DataFrame() Input Pandas dataframe whose index needs to be changed. local_zone : str Name of local zone. Defaults to PST. Returns ------- pd.DataFrame() Dataframe with UTC timestamps. """


        timestamp_new = pd.to_datetime(timestamp, infer_datetime_format=True, errors='coerce')
        timestamp_new = timestamp_new.tz_localize(local_zone).tz_convert(pytz.utc)
        timestamp_new = timestamp_new.strftime('%Y-%m-%d %H:%M:%S')
        return timestamp_new