docs: def get_backend_expiry(self, expiry=DEFAULT_EXPIRY):
          """
          Return the expiry value usable by this backend based upon the provided
          timeout.
          """
          if expiry == DEFAULT_EXPIRY:
              expiry = self.default_expiry
          elif expiry == 0:
              # avoid time.time() related precision issues
              expiry = -1
          return None if expiry is None else time.time() + expiry