It's time that we collected our reports.为什么用过去式?

虚拟语气.该是干什么事的时候了,但这件事现在还没有干,所以是与现实相反的情况,用虚拟语气.即It's time that +过去时句子.
It's time that...中,that从句中的谓语动词用过去时或用"should+动词原形"(should不能略).
例如:
It's time we started.
(It's time we should start.)