Before noon and am (pm) time earlier and now
Often, when visiting other states, especially English-speaking, the indicators of time acquire not a twenty-four, but a twelve-hour format. Only often am and am are added to it.
Time measurements in the old days
In the Middle Ages there was no concept of a single time. In each settlement, a report was kept approximately in this way. The highest point of the sun in the sky meant noon, which in Latin sounded like meridiem (literal translation - middle of the day). All hours until the middle of the day are ante meridiem (a.m.), all hours after - post meridiem (p.m.).
That's what time am / pm literally means. And given that the duodenalized system of calculation prevailed at that time, the time before noon was divided into twelve hours, and the same twelve hours after noon. Thus, the division of the day into twenty-four hours was formed. As is clear from this method of determining the time, in one country the readings of clocks could differ several times.And even if there were fifteen minutes between the cities, in these cities the afternoon or afternoon (am / pm) time differed precisely by these fifteen minutes.
The first to try to solve this problem were English railroad workers in the nineteenth century. The country with the most developed railway network wanted to ensure that the train schedule throughout the country was unified, independent of local time. And they, first of all for their own needs, transferred the schedule of all railways in England to a single London time. Now the train schedule at any station did not depend on what was in the given city, before noon or afternoon (am / pm) time. Twenty years later, the English Parliament, at the legislative level, supported the initiative of the railway workers and introduced a single time record in all of England. A little later, but in the same nineteenth century at an international conference, it was decided to divide the entire territory of the planet into twenty-four time zones and enter a single time in each of these time zones. But they took into account the borders of the states (or territorial units, if the states had large territories).After that, noon ceased to have its value for a particular city, and before noon or afternoon (am / pm) time was equal for cities at a distance of several hundred kilometers. And if the cities were close, but were in different countries, the clock could differ by one or even two hours.
In modern world
In order to transfer twenty-four-hour time to the system “am / pm-time”, it is enough to put the corresponding mode in the phone. Yes, it will be somewhat unusual for perception, but it will exactly meet the standards of literally all English-speaking states.