用过去式是虚拟语气，就好像

"It's time we met."  (我们早该碰面了！）

The verb "met" is in the subjunctive mood.  参见：

http://www.english-for-students.com/Subjunctive-Mood.html

Sentences starting with ‘it is time’ indicate that it is already late for taking the indicated action.

Other Examples: 

• The match will commence at 10o’clock and it is time you got ready. (Not ‘it is time you get ready)
• It is already half past midnight, and it is time we returned home. (Not ‘it is time we return home)

这也就是为什么最后一句有人英语翻成：It is time it was time.