The correct punctuation for the underlined portion of the sentence is:
i promise to be on time ; my alarm is already set.
The semicolon (;) is used to connect two independent clauses that are closely related. In this case, "I promise to be on time" and "my alarm is already set" are two complete thoughts that can stand alone as separate sentences but are related to each other. Using a semicolon helps to indicate the close connection between the two clauses.