It was time of the Olympic Games, so hotel accommodations were scarce. 当时是奥运会比赛期间，所以旅馆住宿很短缺。

It was a time of war between states. 那是个战乱的时期。

It was the time of break and students came out of classroom one after another. 现在是休息时间，学生们一个接一个走出教室。

It was a time of diplomacy. 这是一个外交频繁的时期。

问：为什么从上到下这四个句子中，time前分别不加冠词、加a\an、加the冠词、加a\an，什么时候加或不加，加那个，我实在搞不清楚？