This study analyzed the use of the prepositions "on" and "at" as prepositions of time in argumentative essays written by Malaysian Form 4 and Form 5 students. The study used the Malaysian Corpus on Student's Argumentative Writings (MCSAW) and WordSmith Tools software to identify distribution patterns and common errors. The results showed limited correct use of "on" and "at" as prepositions of time due to the type of essay. Common errors identified included omission, insertion, and selection of incorrect prepositions of time. The study provided insights into students' difficulties with these prepositions.