Talking about time
There is no 24-hour time system in English, rather a 12-hour system. This means that people only count from 0 to 12, not to 24 as they do in some countries. The abbreviations a.m. and p.m. indicate whether a time before or after midday is meant. To indicate that you talk about 10 in the morning, you add a.m. Take for example: "It's 10 a.m." The abbreviation a.m. is Latin and means ante meridiem, which literally means before midday. This covers any time between midnight and midday. When you talk about a time in the afternoon or in the evening, you add p.m. Take for example: "It's 2 p.m." The abbreviation p.m. is Latin and means post meridiem, which literally means after midday.
Usage of abbreviations and time specifications