Do AM and PM Contain Periods?
Summary
Use periods when you lowercase the abbreviations for time (a.m., p.m.); omit the periods when you capitalize them (AM, PM).
- Correct: Farley works from 9:30 a.m. to 6:00 p.m. on weekdays.
- Correct: Farley works from 9:30 AM to 6:00 PM.
Don’t use another period after a.m. or p.m. at the end of a sentence: a single period suffices.
- Incorrect: The conference starts at 11:30 a.m.
Correct: The conference starts at 11:30 a.m.
When to use a.m. and p.m.
The abbreviations a.m. and p.m. are generally used to indicate and emphasize exact time on the clock.
- The train leaves at 10:05 a.m.
- Class starts at 2:15 p.m.
- Office hours are 9 a.m. to 5 p.m.
When the exact time is unimportant, such as in everyday speech, these abbreviations are often omitted, and words like o’clock and a quarter past or half past are used. Using just the numbers is also acceptable in casual communication.
- Rita woke up at 10 o’clock.
- We’ll be there by a quarter past two.
- Anita works from nine to six on weekdays.
In formal writing, use numerals instead of words to refer to time with a.m. and p.m.
- Informal: All the world’s clocks stopped at nine a.m. on Monday.
Formal: All the world’s clocks stopped at 9 a.m. on Monday.
Whether to capitalize or lowercase
In running text, a.m. and p.m. are generally lowercased.
- From 7:30 to 8:15 a.m., we meet to plan the day’s schedule.
- Lulu has lunch from 1:15 to 2:45 p.m.
- We’re going to watch a play today at 6:30 p.m.
These abbreviations may also be capitalized—for example, on digital clocks and in signs, notices, and headings. Whether to capitalize or lowercase is a matter of style rather than grammar.
- Lunch: 11:30 AM to 12:15 PM
sign on a noticeboard
- 7:33 AM
the time on a digital clock
- Fridays: 9 AM to 1 PM
working hours of a clinic
In general, prefer to use lowercase letters for the abbreviations a.m. and p.m. in running text. If you do capitalize them, consider using small caps to improve readability.
- Poor: Farley chewed his nails from 11:55 AM to 12:13 PM.
Better: Farley chewed his nails from 11:55 AM to 12:13 PM.
When to use periods
Use periods after the letters when you lowercase the abbreviations a.m. and p.m. Lowercasing is generally preferred in running text.
- From 1:00 to 1:30 a.m. most days, Poco plays the piano.
- Is it 9 p.m. already?
- At 11:59 p.m., there was a knock on the door.
Omit the periods when you capitalize these abbreviations.
- Hours: 2 AM to 6 PM
- The time is now 2:33 PM. The Durandians will address the planet at 7:30 PM tonight.
Periods are generally omitted in abbreviations that contain capital letters but used in those with lowercase letters.
- MBA, PhD, ATM, DVD, PC, URL, CEO, USA, EU, UNESCO
- e.g., i.e., a.m., et al., ibid., etc.
Many exceptions exist. For a complete discussion, see this article on when periods are used in abbreviations.
Period at end of sentence
Don’t add another period after a.m. and p.m. if these abbreviations appear at the end of a sentence. A single period suffices both for the abbreviation and to end the sentence.
- Incorrect: Your train leaves at 11 a.m.
Correct: Your train leaves at 11 a.m. - Incorrect: The show start at 7 p.m.
Correct: The show starts at 7 p.m.
In British usage, periods (or full stops) are often omitted even in lowercase abbreviations.
- At 4 pm every day, the Queen sits down to tea.
- The train leaves Waterloo Station at 11:15 am.
If you capitalize AM and PM, use a period as usual if they appear at the end of a sentence.
- Correct: Your train leaves at 11 AM.
- Correct: The show starts at 7 PM.