A variety of different styles may be acceptable for formal invitations. The following style should be used in text for print and electronic communications.
Use numbers for times, except for noon and midnight, use a colon to separate hours and minutes and do not use ciphers (double zeros) with whole hours.
The meeting will be held from 8 to 11:45 a.m.
Sessions begin at noon, 2:30 and 4 p.m.
- Lowercase a.m. and p.m. and always use periods.
- Lowercase noon and midnight.
- Do not use 12 noon or 12 midnight (redundant). Use noon or midnight.
- Do not use 12 p.m. or 12 a.m. Use noon or midnight.
- Do not use 8 a.m. in the morning (redundant) Use 8 a.m.
- Do not use o’clock with a.m. or p.m.
Do not use dashes in place of "to" or "through" or "and" or "until" with times of day or days of the week.
Use: The meetings are 8 to 11 a.m., Monday through Thursday.
Not: The meetings are 8-11 a.m., Monday-Thursday.
An n-dash may be used with dates (July 18–21), and should always be used with dates when both days of the week and dates are included.
Use: The play runs Monday through Thursday, July 18–21.
When listing a beginning and ending time separated only by the word "to," or when listing a series of times when all times listed are a.m. or p.m., use a.m. or p.m. only once, following the final time listed.
Use: The meeting will be held from 8 to 11 a.m.
Not: The meeting will be held from 8 a.m. to 11 a.m.
Use: Sessions begin at 8:30, 9:30 and 10:30 a.m.
Not: Sessions begin at 8:30 a.m., 9:30 a.m. and 10:30 a.m.
References to midnight can cause confusion. Is midnight Friday at the beginning or at the end of Friday? That’s why insurance policies usually take effect at 12:01 a.m. and why airline schedules list flights at 11:59 p.m. or 12:01 a.m. and not midnight.
In many references, midnight is perfectly acceptable. In the sentence, "The dance will be held from 8 p.m. to midnight Friday," the meaning is clear.