The new rule is that we cannot work in the office
　　　 in the night after 8:00 p.m.

　　The new rule is that we cannot work in the office at night
　　　 after 8:00 p.m.

　　　　新規章是我們在公司工作不能超過晚上八點。

At night 和 in the evening 是固定用法，不可以混淆。