One day I saw this board "Open to 12:00 AM", I didn't know it said "open to noon" or "open to midnight".
The mainstream opinion is that a.m.=ante meridiem and p.m.=post meridiem. We can see clearly in GMT: Greenwich Mean Time. It says that 12:00am means 12:00 noon, and 12:00pm means 12:00 midnight.
But Chapter 12 of U.S. GOVERNMENT PRINTING OFFICE STYLE MANUAL says that "12 a.m. (noon); 12:15 p.m. (15 minutes past noon). 12 p.m. (midnight); 12:25 a.m. (25 minutes past midnight)".
It confused me. Finally I found the answer in the website of National Institute of Standards and Technology (NIST). It said that "ante meridiem" and "post meridiem" mean "before noon" and "after noon". Noon is neither before nor after noon. So 12 noon and 12 midnight should be used instead of 12 a.m. and 12 p.m. For example, a bank might be open on Saturday from 8 a.m. to noon.
Last, We can read the discuss of "Confusion at noon and midnight" on Wikipedia website.