12 vs. 24

We use A.M. and P.M. (with and without the periods, and with and without the capitals) to determine if it's morning or afternoon/night. This is what's called a 12-hour clock, which means we count the hours up to 12, then we start over. So it wouldn't be 36:30 or 25:30. The most it could be is 12:59, then it goes back to 1:00. The 24 hour clock is widely used today, and in fact most countries in the world use a 24 hour clock, which means that 1:00 in the morning is 1:00 on this clock, but 1:00 in the afteroon is 13:00. We usually refer to this as military time.

So where did the A.M. and the P.M. come from, you ask? Well, they actually come from Latin. A.M. is an abbreviation for ante meridiem which translates to "before midday". P.M. is an abbreviation for post meridiem which translates to "after midday". Noon is meridiem. So, technically, 11:59 is A.M. and 12:01 is P.M., and 12:00 noon is merdiem. We use the term "afternoon" to literally mean the time after the noon hour, which is like a modern translation of post meridiem.

So you may be wondering, if a 24 hour clock can start at 00:00 at midnight, then 12:00 would be noon, then what would the following midnight be? Would it be 24:00? Or would it start over at 00:00? The answer is both. It depends on what method is being used. Most would consider 00:00 to be the start of day 1 and 24:00 to be the end of day 1. But others would consider 00:00 to be the start of day 1, and the last second of day 1 to be 23:59:59, and when it rolls over, it goes back to 00:00 for the start of day 2. Typically, if you're only talking about one day, then you could use 24:00, but if you're using it for time always or if you're referring to more than one day, then you would probably be more accurate using 00:00 on day 2's beginning.

The 12 hour clock can be traced back to Mesopotamia and ancient Egypt (around 1500 BC). The Romans used the A.M. and P.M., with midnight being halfway between sunrise and sunset (so it changed from season to season), hence the name literally meaning "middle of the night". The main difference in the Romans A.M. and P.M. was that it was backwards from ours. If they said it was 5 A.M., it meant it was 5 hours before meridiem (for us 7 A.M.), and if we say it's 5 A.M., it means that it's 5 hours into the morning, or 5 hours past midnight.

By the way, on some analog clocks around the world that follow the 24 hour clock, there are two sets of numbers: the number 12 is paired with either a 00 or 24, and 1-11 is paired with 13-23 respectively. This way, someone who's used to the 12 hour clock can read it, and someone who's used to the 24 hour clock can read it.

Bet you didn't know that!