'Convert a column of a dataframe to UNIX timestamps if applicable
:param col:     A Series object representing a column of a dataframe.'

@classmethod
def _to_timestamp(cls, col):

    try:
        col = pd.to_datetime(col)
    except ValueError:
        logging.warning('Could    not    convert    field    to    timestamps:    {0}'.format(col.name))
        return col
    converted = []
    for i in col:
        try:
            converted.append(i.timestamp())
        except ValueError:
            converted.append(pd.np.NaN)
        except AttributeError:
            converted.append(pd.np.NaN)
    return pd.Series(converted, index=col.index)
