code: def _timestamp_query_param_from_json(value, field):
      
      if _not_null(value, field):
          # Canonical formats for timestamps in BigQuery are flexible. See:
          # g.co/cloud/bigquery/docs/reference/standard-sql/data-types#timestamp-type
          # The separator between the date and time can be 'T' or ' '.
          value = value.replace(" ", "T", 1)
          # The UTC timezone may be formatted as Z or +00:00.
          value = value.replace("Z", "")
          value = value.replace("+00:00", "")
  
          if "." in value:
              # YYYY-MM-DDTHH:MM:SS.ffffff
              return datetime.datetime.strptime(value, _RFC3339_MICROS_NO_ZULU).replace(
                  tzinfo=UTC
              )
          else:
              # YYYY-MM-DDTHH:MM:SS
              return datetime.datetime.strptime(value, _RFC3339_NO_FRACTION).replace(
                  tzinfo=UTC
              )
      else:
          return None
docs: Coerce 'value' to a datetime, if set or not nullable.
  
      Args:
          value (str): The timestamp.
          field (.SchemaField): The field corresponding to the value.
  
      Returns:
          Optional[datetime.datetime]: The parsed datetime object from
          ``value`` if the ``field`` is not null (otherwise it is
          :data:`None`).