docs: def time_description(self):
          """String description of the year or year range"""
  
          tc = [t for t in self._p.time_coverage if t]
  
          if not tc:
              return ''
  
          mn = min(tc)
          mx = max(tc)
  
          if not mn and not mx:
              return ''
          elif mn == mx:
              return mn
          else:
              return "{} to {}".format(mn, mx)