>>16702945AM means before noon. PM means after noon. Noon means when the Sun is at its highest point in the sky for that day. Though modern convention is that 12:00 "noon" is PM, actual astronomical noon is neither AM nor PM.
We've abandoned all of that for convenience and commerce. Plus a bit of idiocy. Daylight Saving Time makes things even more out of alignment with the scientific standard, which is why winter is considered standard and the majority of the year is not.
As a side note, by convention we think of midnight as 12:00 AM but the instant of midnight, like noon, is neither AM nor PM. It's not even part of any specific day. It's the division between two days, not part of either. Simply calling it 12:00 AM is much more convenient for our digital systems.