code: def to_timestamp(self, freq=None, how='start', axis=0, copy=True):
          
          new_data = self._data
          if copy:
              new_data = new_data.copy()
  
          axis = self._get_axis_number(axis)
          if axis == 0:
              new_data.set_axis(1, self.index.to_timestamp(freq=freq, how=how))
          elif axis == 1:
              new_data.set_axis(0, self.columns.to_timestamp(freq=freq, how=how))
          else:  # pragma: no cover
              raise AssertionError('Axis must be 0 or 1. Got {ax!s}'.format(
                  ax=axis))
  
          return self._constructor(new_data)
docs: Cast to DatetimeIndex of timestamps, at *beginning* of period.
  
          Parameters
          ----------
          freq : str, default frequency of PeriodIndex
              Desired frequency.
          how : {'s', 'e', 'start', 'end'}
              Convention for converting period to timestamp; start of period
              vs. end.
          axis : {0 or 'index', 1 or 'columns'}, default 0
              The axis to convert (the index by default).
          copy : bool, default True
              If False then underlying input data is not copied.
  
          Returns
          -------
          DataFrame with DatetimeIndex