docs: def check_cache(path):
      """Return true if the cache file holding list of all datasets
      does not exist or is older than 30 days
      """
      if not os.path.exists(path):
          return True
      else:
          # check the age
          mod_date = datetime.fromtimestamp(os.path.getmtime(path))
          if mod_date < (datetime.now() - timedelta(days=30)):
              return True
          else:
              return False