General Rules:

  • Use a.m. and p.m. for time listings whenever possible. Using a time construction that includes “o’clock” (5 o’clock) is acceptable, but a.m. and p.m. are preferred.
  • In general, spell out the number of hours, minutes, days, years, etc. when less than 10.
  • Use official names for time periods when it’s applicable.

Hours, Minutes, Seconds; Days, Months, Years


Rule: Spell out numbers less than 10 when preceding hours, minutes, seconds, days, months, years, etc.

  • Example: He worked for eight hours.
  • Example: The referee announced a two-minute warning.
  • Example: The famine lasted five years.

Historical Periods of Time


Widely recognized epochs
Rule: Anthropology, archaeology, geology and history all speak of historical periods of time with official names. Capitalize these as proper nouns.
  • Example: the Bronze Age
  • Example: the Dark Ages
  • Example: the Middle Ages

Widely recognized periods and events
Rule: Capitalize those with official names.
  • Example: the Boston Tea Party
  • Example: the Great Depression
  • Example: Prohibition

General time period descriptions
Rule: Capitalize proper nouns and proper adjectives only in descriptions of general periods.
  • Example: classical Rome
  • Example: the Victorian era

Century
Rule: Use ordinal numbers for 10 and above; spell out numerals for nine and below. Use lowercase letters.
  • Example: the sixth century
  • Example: the 19th century
  • Example: mid-19th century

    Note: Use the official AP Stylebook for capitalization for historical periods of time not found here, or use Webster’s New World College Dictionary, Fourth Edition as a reference. If Webster’s uses lowercase for the sense in which the word is used, then use lowercase.

    Time of Day


    Rule: Use figures for all times of day except noon and midnight, and separate hours from minutes with a colon. Use periods for a.m. and p.m. abbreviations.
    • Example: 10 a.m.
    • Example: 3 p.m.
    • Example: 4:30 p.m.

    Midnight, noon

    Rule: Use midnight and noon, not 12 a.m. and 12 p.m.

    • Correct: The birthday party starts at noon and ends at midnight.
    • Incorrect: The party starts at 12 p.m. and ends at 12 a.m.
    Time spans
    Rule: If both times are in either the a.m. or p.m., use a hyphen between the figures, and include the abbreviation only once. If one time is a.m. and the other is p.m., use “to” between the times while using the abbreviations for each.
    • Example: 2-5 p.m.
    • Example: 8 a.m. to 4 p.m.

    Redundancies
    Rule: Omit “morning” and “tonight” when using a.m. and p.m.
    • Correct: 8 a.m.
    • Correct: 8 p.m.
    • Correct: 8 a.m. Tuesday
    • Incorrect: 8 a.m. this morning
    • Incorrect: 8 p.m. tonight
    • Incorrect: 8 a.m. Tuesday morning