这其实是一个句型,只不过后面要跟不同的东西.
It's time to跟的是不定式（to do）. 也就是说,要跟一个动词,意思是“到做某事的时候了”.如：
It's time to go home.
It's time to tell him the truth.
It's time for 跟的是名词.也就是说,不能跟动词.如：
It's time for lunch. (没必要说It's time to have lunch)
It's time for class. (没必要说It's time to begin the class.)
总之,句子能短则短.能省一个词就省一个词.用一个名词就解决的问题,就不用to 加动词了.