A really interesting question! Both could be correct, depending on the situation. 1. Addressing a person: You are late (Harry). "You almost missed the bus" something that might have happened in the past but did not, so the past tense is used. 2. A narrative in the present tense: You might be talking about yourself or another person. You (yourself or another person) are late. You almost miss the bus, but are lucky and get to work on time.