Telling the Time

We measure the time in seconds, minutes and hours.

There are 24 hours in a day.  There are 60 minutes in an hour. There are 60 seconds in a minute.

To tell the time you usually use a clock or a watch.

A clock:  A watch: 

There are analogue clocks and watches and there are digital clocks and watches.

(...)

Analogue clocks and watches often have numbers on the "face" to show the hours and lines to show the minutes. An analogue clock also has "hands", a short hand to show the hour, a longer hand to show the minutes, and sometimes a long thin hand to show the seconds.	14:00 Digital clocks and watches show the exact hours and minutes in numbers. They use 24 hours and sixty minutes to show the time.

Telling the time to the half hour or quarter hour

There are 15 minutes in quarter of an hour.

There are 30 minutes in half an hour.

There are 45 minutes in three quarters of an hour.

When it's past the hour (up to 30 minutes past) we say "past".

When it's before the hour (after 30 minutes past) we say "to".

30 minutes is half an hour, we say "half past" or "thirty".

15 minutes is quarter of an hour. At 15 minutes past the hour we say "quarter past" or "fifteen". At fifteen minutes to the hour we say "quarter to" or "forty-five".

Twelve o'clock

Twelve fifteen or  Quarter past twelve

Twelve thirty or Half past twelve

Twelve forty-five or Quarter to one

Note - you don't mention the "minutes".

Writing the time

AM	00:01 - 11:59
write a.m. - stands for Ante Meridiem (the time between midnight and noon)	00:01 to 11:59
write noon or midday	12:00
write p.m. - stands for Post Meridian (after noon)	12:01 - 23:59
write midnight	24:00

Speaking about the time generally

If you say "Good morning" at 1 am, people will think you are strange. Different rules apply, which are more to do with how light it is. See also "Greetings".

Daytime - the time between sunrise and sunset	Starts at sunrise / dawn
morning - the time between sunrise and noon	sunrise - 11:59
afternoon - the time between noon and sunset	12:01 - sunset
Nighttime - the time between sunset and sunrise	Starts at sunset / dusk
evening - the time between sunset and night (full darkness)	sunset - darkness
night - the time after evening and before sunrise	dark - sunrise

There are 24 hours in a day, but only the military, police and computer programmers use the 24-hour clock. When writing or speaking generally we tend to use the 12-hour clock.

The 24 hours of the day are divided into two periods called a.m. (Latin "ante meridiem" | English: "before mid day") and p.m. (Latin "post meridiem" | English: "after mid day").

The way people write the time varies. I prefer a.m. and p.m.

Choose from the following styles or use what your English teacher tells you to and stick to it:-

a.m. p.m.
am pm
AM PM
A.M. P.M.

Some people use a dot as the separator: 2.30 pm.

Some people use a colon as the separator: 2:30 pm. The colon is usually used with the 24-hour clock: 14:30.

http://www.learn-english-online.org/Lesson17/Lesson17.htm