Milhouse,
1. According to the Cambridge English Dictionary “stop by” means “to go into a place for a short time when you are going somewhere else”.The Oxford English Dictionary only says “to visit briefly”. “By” here is apart of the verb. This is known as a phrasal verb.
so you are saying that midnight is both 12AM and 12PM. However, The American Heritage Dictionary of the English Language (quoted by the Wikipedia) states “By convention, 12 AM denotes midnight and 12 PM denotes noon.