AM PM

1. 24-hour clock system

00:00:00 - 23:59:59

2. 12-hour clock system

12:00:00 am - 11:59:59 am

12:00:00 pm - 11:59:59 pm

no 00:00 am or 00:00 pm, it should be 12 midnight or 12 noon

It is incorrect to write 0:00p.m. The use of 0:00 implies a 24-hour clock system, while the use of A.M. and P.M. implies a 12-hour clock system. Therefore, it is absolutely wrong to use both together.

The 12-hour clock divides the day into two parts, 12 hours each. The middle of the day (midday) is the twelfth hour, noon, or the meridiem. A.M. refers to the hours prior to midday (Latin: ante meridiem) while P.M. refers to the hours after midday (Latin: post meridiem). Since the meridien is thought to occur right at the exact second of the 12th and 24th hours, it is most common to hear the times referred to as 12 noon or 12 midnight, rather than using a.m. or p.m. However, to be clear, 12:01p.m. refers to the time just after (post) the 12th hour, that is, the first minute after midday (noon); while 12:01a.m. refers to the first minute of the new day, therefore prior to (ante) midday, that is, just after midnight (after the 24th hour of the previous day).


The 24-hour clock does not need a reference to the middle of the day, the meridiem, which is why the suffix a.m. or p.m. used with this system is completely unnecessary, incorrect, and confusing. So you are wrong to say that “Okay, 0:01AM means something”. It actually doesn’t mean anything at all for the reasons I have given. 0:01 as written implies the 24-hour clock (although it would probably be written 00:01 instead). It refers to the start of the new day. The equivalent in the 12-hour clock would be 12:01 midnight (or 12:01a.m.). 


So if your friends ask you to meet up at 12:00a.m., it should be very clear to you by now that you would be left standing all alone and feeling silly if you showed up at midday, noon, 12:01 p.m., the 12th hour… They are asking you to show up at the first hour of the day, which is just after (or right at) midnight.