Telling the time in English

The word o'clock is used only with exact hours.

We use the word minutes with the figures which can be divided by 5 and in the result don't give a full number.

In American English we use after instead of past and of instead of to.

Write Say
10.00 in everyday speech:
It's ten o'clock.
Ten. (very informal)
It's ten.
10.05 It's five past ten.
(It's) five past (where the hour is known)
10.15 (a) quarter (NOT: fifteen) past (10)
10.35 (It's) twenty-five to (eleven)
9.57 three minutes to ten
10.38 twenty-two minutes to eleven
10.45 AmE. a quarter of eleven
10.15 AmE. a quarter after ten
10.30 half ten (very informal)
10.15 ten fifteen (very informal)
10.30 ten thirty (very informal)
a.m. = ante meridiem / before midday
p.m. = post meridiem / after midday
12 a.m. at noon
12 p.m. at midnight

The time in schedules and timetables:

Write Read
09.00 nine hundred hours
09.03 nine oh three
09.10 nine ten
09.15 nine fifteen
9.30 nine thirty
09.36 nine thirty-six
09.45 nine forty-five
21.00 twenty-one hundred hours
21.03 twenty one oh three
21.10 twenty-one ten
21.15 twenty-one fifteen
21.30 twenty-one thirty
21.36 twenty-one thirty six
21.45 twenty-one forty-five