O’Clock: Whole, Quarter, and Half Hours
Summary
Use o’clock to indicate the hour on the clock: specify only the hour and not the minute. You may use either numerals or words (ten o’clock or 10 o’clock). Using o’clock is common in informal communication when the exact time is unimportant.
The phrases a quarter to and a quarter past are used to indicate times 15 minutes before or after the hour (a quarter to three for 2:45, a quarter past three for 3:15). The time 30 minutes past the hour is shown by half past (half past three for 3:30).
In formal and official communication, when emphasizing exact time is important, use a.m. and p.m. (10 a.m., 3:15 p.m.) or the 24-hour clock (10:00, 15:00).
Meaning of o’clock
The term o’clock, which means “according to the clock,” is a contraction of the phrase of the clock.
- Our train leaves at nine o’clock tomorrow morning.
nine of the clock
- Is it 8 o’clock already?
the time according to the clock
When to use o’clock
Use o’clock to indicate a specific hour on the clock. This term is often used in everyday speech and informal writing, where talking about exact time down to the minute is unimportant. (When exact time is important, use a.m. and p.m. instead.)
- Poco practices playing the piano at four o’clock every morning.
- Poor Farley, who lives right above Poco, has class at nine o’clock.
- Let’s meet at 1 o’clock for lunch.
Only the hour is shown with o’clock, so use it when you want to refer to the hour but not the minute.
- Incorrect: We have tea at four thirty o’clock.
Correct: We have tea at four o’clock.
Correct: We have tea at four thirty.
Numerals vs. words with o’clock
You may use either numerals or words with o’clock. Style guides differ in their recommendations. The Chicago Manual of Style suggests using words; the AP Stylebook recommends numerals. Stay consistent in usage across a document.
- I have pottery class at four o’clock / 4 o’clock today.
- Maya’s train leaves at two o’clock / 2 o’clock this afternoon.
- The party is at eight o’clock / 8 o’clock tonight.
You may use either numerals or words with o’clock, but never insert a space after the apostrophe in the contraction.
- Incorrect: At 12 o’ clock, there was a knock on the door.
Correct: At 12 o’clock, there was a knock on the door.
Quarter and half hours
Quarter and half hours on the clock are usually referred to by words rather than numerals: this style is common in informal and creative writing. To refer to 15 minutes before the hour, use the phrase a quarter to or a quarter of. Similarly, to indicate 15 or 30 minutes past the hour, we say the time is a quarter past or half past the hour.
- Could you call me at a quarter to eleven?
ten forty-five
- We’re open till a quarter of seven today.
six forty-five
- We break for lunch at a quarter past one.
one fifteen
- There’s a ferry every morning at half past nine.
nine thirty
The article “a” in a quarter to or a quarter past is optional.
- Lunch is served at quarter to one (or a quarter to one).
O’clock vs. a.m./p.m.
In official and formal writing, prefer specifying a.m. or p.m. rather than using o’clock. Similarly, in formal contexts, prefer using numerals over words to indicate quarter and half hours.
- Informal: Don’t forget to call him at five o’clock.
Formal: Please call me back at 5 p.m. - Informal: It was a quarter to nine, and we were starving.
Formal: Your reservation is for 8:45 p.m. today. - Informal: At half past nine, the phone rang.
Formal: The interviewer called me at 9:30 a.m.
The abbreviations a.m. and p.m. are generally lowercased in running text, with periods between the letters. In headings and notices, however, they are often capitalized. The periods are then omitted.
The 24-hour system is also used to emphasize exact time. This usage is common in military settings, hospitals, airports, train and bus stations, and research labs.
- Boarding is at 20:00, and departure at 20:30.
At an airport, indicating exact time is preferable to using phrases like “eight o’clock” or “half past nine.”
- We took off from Bogota at 0600 hours.
But in everyday speech and informal usage, where specifying the exact time isn’t important, the term o’clock and phrases like quarter past and half past sound more natural than using a.m. and p.m. or the 24-hour clock.