Mystery

A: What time does our train leave?
B: In 20 minutes. It's time we (        ) for the station.

1. left
2. have left
3. had left
4. will have left

Which one's correct?
This is one of question in Grade 2 of Eiken ...

The answer is 1. "left"
Why????

Please teach me why I have to select "left" !!