The 12-hour clock with am and pm is standard in the United States, Canada, Australia, the Philippines and much of the English-speaking world. Most of Europe, Latin America and Asia writes the 24-hour clock, where the afternoon simply continues from 13:00 to 23:59.
The 24-hour clock removes an entire class of error: there is no way to confuse 07:00 with 19:00. That is why it is used for transport, medicine and military operations everywhere, including in countries that otherwise use am and pm.
The midnight trap
"12am" is the beginning of the day and "12pm" is midday, but the labels are unintuitive enough that people regularly book the wrong one. If a time near midnight matters, write 00:00 or 23:59 instead.