It's (high) time that sb. did (should) do sth.和It's (high) time (for sb.) to do sth.都表示“某人该做某事了”，它们在意义上几乎没有什么差别。至于It's (high) time that...为什么要用虚拟语气，连老外都说不清，我觉得没有必要去深究，会用既可。这是一个很常见的表达，英美国家人们都这么用，所以建议你也无需过于纠结。对于这类句式，《葛传槼英语惯用法词典》（P.618-619）有几点说明，摘录如下（为了更容易比较，我在每句前都加了序号，解说中也相应地改用该序号）：

12.下面每组里除了第一句以外都有“我们不该再延迟”的含意：

①It is time for us to start.

②It is high time for us to start.

③It is time we started.

④It is high time we started.

⑤It is time that we started.

⑥It is high time that we started.

⑦It is time we should start.

⑧It is high time we should start.

⑨It is time that we should start.

⑩It is high time that we should start.

①It was time for us to start.

②It was high time for us to start.

③It was time we started.

④It was high time we started.

⑤It was time that we started.

⑥It was high time that we started.

⑦It was time we should start.

⑧It was high time we should start.

⑨It was time that we should start.

⑩It was high time that we should start.

下面几点该注意：

（1）time前没有a货the。

（2）在口语里往往省去It is或It was。

（3）用high不及不用high普通。

（4）用that不及不用that普通。

（5）should start不及started普通。

（6）第一组里的③、④、⑤、⑥各句里的started改作start也通，但远不及started普通。

（7）每组里的③、④、⑤、⑥里的started原来属于虚拟过去式，但现在通常用陈述过去式，如通常说It is (或It was) (high) time (that) I was told about the matter,不说It is (或It was) (high) time (that) I were told about the matter。当然，在we were, we started, I started等表达方式里的were和started在虚拟过去式和陈述过去式里形式相同。

（8）每组里的③、④、⑤、⑥句里的started改作were starting也可以，但很不普通。

（9）第一组里的③、④、⑤、⑥里用started，在形式上是过去时态，在事实上却指未来。说这四句的任何一句的时候实际还不曾start。这种结构似乎很奇特，但实际是很普通也很纯正的英语，你在读书的时候见到它，别以为那属于过去时态的动词是印错的。

（10）下面两句意思相同：

It's getting time we started.（我们该动身的时候快到了。）

It's about time we started.