The question you should be asking is “why is the first hour of each half-day assigned the value 12 instead of 0? With any other kind of measurement we start at 0; why not with time?

Time designation does start with zero, almost everywhere in the world except North America. Somehow it isthought it's a good idea to have a system where 12:30 PM is earler than 11:30 PM. Elsewhere, Noon is just 12:00. Midnight is 00:00. No confusion, no arithmetic anomalies.

Now you know.