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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 fit(self, col):
 """Prepare the transformer to convert data. Args: col(pandas.DataFrame):
 Data to transform. Returns: None """

        dates = self.safe_datetime_cast(col)
        self.default_val = dates.groupby(dates).count().index[0].timestamp() * 1e9