1. In time, Mr. Fleming's son graduated from a top medical school in London.

(What is the meaning of "In time"? What other expressions can we use?)

2. You have to come to the party in time.
(What about 'in time' here? What is the difference of 'in time' between in Sentence 1 and Sentence 2?)

"In time" means "eventually" or "at an unspecified time in the futute."

http://www.answers.com/eventually
You could also say, "Eventually" or "After several years."

2. The phrase "in time" is not used correctly. It should be "on time" meaning at the correct time -- not late!

1. In the sentence "In time, Mr. Fleming's son graduated from a top medical school in London," the phrase "in time" means that it took a certain amount of time for Mr. Fleming's son to graduate. It implies that a significant period of time passed before the event occurred. Other expressions that convey a similar meaning are "eventually," "over time," or "after a while." Using any of these phrases would indicate that the graduation did not happen immediately, but after a certain duration.

2. In the sentence "You have to come to the party in time," the phrase "in time" has a different meaning compared to the first sentence. Here, it means that you need to arrive at the party at the designated or expected time. It suggests being punctual and not arriving late. In this context, "in time" is the opposite of being late. Other phrases that can convey a similar meaning are "on time" or "before the appointed time." These expressions emphasize the importance of being prompt and not missing a specific deadline or event.