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Questions and answers

Is midnight
12am or 12pm?

What is this image?

Is it midday, noon or midnight? 

We all know that noon or midday would be a nice time for lunch, and midnight means it’s probably time for bed. This is why it’s always best to use these words instead of saying ‘I’ll meet you at 12’. Not too many venues are still serving lunch by midnight!

Is midnight today or tomorrow?

To avoid confusion, it is always better to use the 24-hour clock, so that 12:00 means noon or midday.

There is still disagreement and confusion about whether 00:00 is ‘today’ or ‘tomorrow’, but it is generally accepted that the moment the clock hits midnight marks the start of a new day. This means that 23:59:59 would be the last second of ‘today’, and the moment the clock hits 00:00, the new day has begun. This is why we celebrate the turning of the new year at the strike of midnight, and Cinderella was issued a strict warning about midnight in the popular fairy tale. It is common for transport timetables to use 23:59 or 00:01 to make it really clear which day they mean – same goes for deadlines on assignments and forms. 

See more Questions and answers

Midday and midnight

There are no official standards established for the meaning of 12am and 12pm, but it is generally accepted that 12am means midnight and 12pm means midday.  The ‘am’ and ‘pm’ stand for ante meridiem and post meridiem – Latin words meaning ‘before the middle of the day’ and ‘after the middle of the day’.

When the clock hits 12:00:00, you have hit ‘meridiem’. So all times before that are ‘ante meridiem’ within the same day, and all times afterwards are ‘post meridiem’, up until the clock hits midnight and a new day begins.

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.

To avoid confusing matters, it is always best to use the words midday, noon and midnight.

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.

Learn more about our work with time

As the UK's National Metrology 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. Accurate time keeping is not just important to make sure you meet you friends for lunch on time – it is also used to accurately timestamp financial transactions in trading, keep our GPS systems running, and even has implications for the UK’s national security.

NPL's research on time

GPS common-view time

Internet time from NPL

MSF radio signal

NPLTime

National Timing Centre programme

Led by NPL, the UK’s first nationally distributed time infrastructure will improve security and resilience, communication, and implementation of new technologies, and pave the way for trusted time and frequency across the country.

Learn more about NTC
 

This is a next generation (trapped ion) atomic clock

Atomic clocks underpin many technologies that are part of our daily lives, such as mobile phones, the internet and global navigation satellite systems. NPL is building the next generation of atomic clocks using laser‑cooled trapped ions or atoms, with which we should achieve accuracies around 100 times better than currently possible. This is equivalent to gaining or losing no more than one second in the age of the universe.

Learn more about optical atomic clocks