'Add a new timestamp element.
Arguments:
timestamp  -- The timestamp in ISO format.'

def add_timestamp(self, timestamp, substanzas=None):

    if (timestamp not in self._timestamps):
        self._timestamps.add(timestamp)
        ts = Timestamp(parent=self)
        ts['value'] = timestamp
        if (not (substanzas is None)):
            ts.set_datas(substanzas)
        self.iterables.append(ts)
        return ts
    return None
