which의 쓰임 질문입니다.

jacson said the blackout lasted several minutes, during which time he felt no pain, even though he had fallen and hurt himself very badly.
위 문장에서 which는 blackout을 대신하는 관계대명사인가요?
그렇다면 뒤의time은 왜 따로 씌어있나요? during which he felt ~~~이게 맞는거 아닌지요?
또 that으로 바꿔도 상관없나요?