Is midnight 12 a.m. or 12 p.m.? (FAQ - Time)
There is no confusion when using the words 12 noon (or midday) and 12 midnight, although the use of 12 midnight can raise the question of 'which day?'. To avoid confusion in, for example, an insurance certificate, it is always better to use the 24-hour clock, when 12:00 is 12 noon and, for example, 24:00 Sunday or 00:00 Monday both mean 12 midnight Sunday/Monday. It is common in transport timetables to use 23:59 Sunday or 00:01 Monday (in this example), or 11:59 p.m. or 12:01 a.m., to further reduce confusion.
There are no standards established for the meaning of 12 a.m. and 12 p.m. It is often said that 12 a.m. Monday is midnight on Monday morning and 12 p.m. is midday. This puts all the times beginning with 12 and ending with a.m. in the same one-hour block, similarly with those ending with p.m. It can also be argued that by the time you have seen a clock showing 12:00 at mid-day it is already post meridiem, and similarly at midnight it is already ante meridiem. Times in the first hour of the day are sometimes given as, for example, 00:47 a.m., with 00:00 a.m. corresponding to midnight, but with a time twelve hours later given as 12:47 p.m.
Another convention sometimes used is that, since 12 noon is by definition neither ante meridiem (before noon) nor post meridiem (after noon), then 12 a.m. refers to midnight at the start of the specified day (00:00) and 12 p.m. to midnight at the end of that day (24:00). Given this ambiguity, the terms 12 a.m. and 12 p.m. should be avoided.
As the UK's National Measurement Institute, the National Physical Laboratory (NPL) has a vital role to play in maintaining the UK's time scale. The current atomic clock system at NPL is the basis of all UK time, and cutting-edge research is being carried out to improve timekeeping accuracy even further.