in是指在...之内，所以不难理解in time可以是指提前完成某事，是及时；on则是指在某一时间点上，是个确切的时间点，所以on time是指准时。

in time的例句

Tom managed to get to the airport in time.

汤姆设法及时赶到了机场。

There's a slight chance that he'll be back in time.

他及时赶回来的可能性不大。

I hope we get there in time.

我希望我们及时到达那里。

You should return the book in time.

你应该及时还书。

It's good to go to school on time.

按时上学是好的。

on time的例句

She always pays her bills on time.

她总是按时支付账单。

We worked like crazy to get it done on time.

我们拼命地干，好按时完成这项工作。

The bus came right on time.

公共汽车正好准时到达。