Time

The 12-hour-system in English

In English there is no 24-hour-system .

15:00 is 3 o'clock (p.m.) (on TV: fifteenhundred). That's why you use a.m.  and p.m. in situations where it is necessary to point out that you want to say in the morning or in the evening. In other situations (or when it is clear that school starts in the morning and the party in the evening) it is left out.

 When to use o'clock

o'clock (abbreviation: of the clock) only on the hour

 Quarter past and to

quarter → 15 minutes
half → 30 minutes

 The minutes

multiplies of 5: leave out the word minutes (but: 6.01 → It's one minute past 6.)