One thing that annoys me about am/pm is that the clocks have a 12 at the top and not a 0. So it goes 11:59 am to 12:00 pm and not 11:59 am to 00:00 pm. Even as a kid I was confused as to why 12 belonged to PM but 11 was still AM, but I never thought about it much until recently, decades after I had learned and was using the system. Which I guess is why it is still like this.