Telling time is very important in day-to-day communication, for establishing a meeting with someone, making an appointment, and talking about your routine, among other functions.
In English, there is Regular (12-hour) Time and Military (24-hour) Time.
REGULAR (12-hour) TIME is used by most people in everyday talk.
If you want to express the exact hour, use “o’clock”. For example, “It’s one o’clock” (1:00). Or “It’s four o’clock” (4:00). You can only say “o’clock” at the exact hour.