code: def expire_at(self, key, expiry, **opts):
          
          key, store = self._expand_opts(key, opts)
          data = store.get(key)
          if data is not None:
              data = list(data)
              data[EXPIRY_INDEX] = expiry
              store[key] = tuple(data)
          else:
              raise KeyError(key)
docs: Set the explicit unix expiry time of a key.