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Questions and answers

Is midnight
12am or 12pm?

Are the words noon, midday and midnight clearly understood?

There is no confusion when using the words 12 noon (or midday) and 12 midnight, although the use of 12 midnight can raise the questions
Is midnight today or tomorrow?
What does midnight mean?

To avoid confusion, it is always better to use the 24-hour clock, so that 12:00 is 12 noon. Therefore 24:00 Sunday or 00:00 Monday are both midnight meaning Sunday to Monday.

It is common for transport timetables to use 23:59 Sunday or 00:01 Monday, or 11:59 pm or 12:01 am, to further reduce confusion.

Midday and midnight conventions

There are no standards established for the meaning of 12am and 12pm. It is often said that 12am Monday is midnight on Monday morning and 12pm is midday. This puts all the times beginning with 12 and ending with am in the same one-hour block, similarly with those ending with pm.

It can also be argued that by the time you have seen a clock showing 12:00 at midday 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 am, with 00:00 am corresponding to midnight, but with a time twelve hours later given as 12:47 pm.

Another convention sometimes used is that, since 12 noon is by definition neither ante meridiem (before noon) nor post meridiem (after noon), then 12am refers to midnight at the start of the specified day (00:00) and 12pm to midnight at the end of that day (24:00). Given this ambiguity, the terms 12 am or 12 pm should be avoided in order to provide a simple definition of midnight.

NPL's work with time

As the UK's National Measurement Institute, we have 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.

 

Learn more about our research on time

Learn more about our time services

Learn more about our NPLTime

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