The abbreviations AM and PM can be found on watches with digital display in the 12-hour display as well as on flight plans and timetables of all kinds abroad. It is particularly widespread in the USA. There the "twelve-hour system" is usual, so in the afternoon one does not speak of 16 o'clock, but of 4 o'clock. AM and PM are used to distinguish between day and night. It is therefore said whether one speaks of morning/morning or afternoon/evening. But what does that mean?
The two abbreviations come from Latin: AM (Ante Meridiem): means "before noon", PM (Post Meridiem): means "after noon". There are different spellings, like e.g: AM and PM, a.m. and p.m., A.M. and P.M., A/M and P/M or AM / PM.
Especially in English speaking countries like Australia, New Zealand, Canada or the USA the 12-hour-count is used. In Great Britain, however, not, there as everywhere in Europe (and also in Germany) the 24-hour-counting (0:00 to 23:59 o'clock) is used (which is also called military time).
In the 24-hour count, 12:01 noon is followed by 12:01 noon and 13:00 on the hour. At night, there is the special feature in the 24-hour count that 00:00 follows 23:59 (in the 12-hour count, the change from AM to PM and vice versa takes place at 12:00, but the count continues from 12:00 to 12:59, then 1:00).
Not only with timetables, but also with appointments or dates of all kinds one must change in countries with 12-hour counting. A clock with a digital display is helpful, because you can easily change here. This is also possible with the smartphone, the Smartwatch or the tablet or laptop.
The 12-hour count goes back to the Middle Ages. The day was originally divided into two halves (light and dark or day and night), which was rather inaccurate.
With the invention of the clock, which appeared to the population mainly in the form of the tower clock at the churches, 12-hour counting was introduced. Because of the greater clarity, the 24-hour counting has become established in the administration as well as in the economy, which today is standard throughout Europe and many other parts of the world. In Germany, even the spelling of office and administrative hours is regulated in a DIN standard (5008).
On watches with digital display it is generally possible to switch between 12- and 24-hour display. Watches with analog display are typically in 12-hour mode, but there are also watches with 24-hour display (e.g. slow watches) or an extra hand for the 24-hour display on the outer ring of the dial.