答：in time表示“及时”,指在约定的时间之前发生.
on time表示“准时、按时”,指正好在约定的时间发生.如：
We hope you will arrive in time to attend the meeting. 我希望你能及时赶来参加这个会议.
They were just in time for the bus. 他们正好赶上了汽车.
The train came on time. 火车正点到站.
I’ll write to your father if you aren’t here on time tomorrow. 倘若你明天不准时到的话,我就要写信告诉你父亲.