code: def frost_days(tasmin, freq='YS'):
      r
      tu = units.parse_units(tasmin.attrs['units'].replace('-', '**-'))
      fu = 'degC'
      frz = 0
      if fu != tu:
          frz = units.convert(frz, fu, tu)
      f = (tasmin < frz) * 1
      return f.resample(time=freq).sum(dim='time')
docs: r"""Frost days index
  
      Number of days where daily minimum temperatures are below 0℃.
  
      Parameters
      ----------
      tasmin : xarray.DataArray
        Minimum daily temperature [℃] or [K]
      freq : str, optional
        Resampling frequency
  
      Returns
      -------
      xarray.DataArray
        Frost days index.
  
      Notes
      -----
      Let :math:`TN_{ij}` be the daily minimum temperature at day :math:`i` of period :math:`j`. Then
      counted is the number of days where:
  
      .. math::
  
          TN_{ij} < 0℃