12 noon is 12 pm
am and pm stand for ante meridian and post meridian (if I'm spelling it right) respectively which basically allude to the sun being before or after the halfway point in the sky. So 12 noon (by convention, if not 100% accurate - and I have no fucking idea if it is or not) is considered the crossover point, so it becomes pm. 12 midnight is 12:00 am 12 noon is 12:00 pm
if we are working on a 24 hour clock aka military time, there is technically no 24:00 - midnight is represented as 00:00 hrs
Years get more interesting. Since there was no year 0 as such then it started with 1AD (anno domini - in the year of our lord) though not everywhere adheres to this - hence 2021 was in fact the first year of the third (edited: I accidentally typed second) decade of this millenia (or is it millenium?) not 2020
