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