The Editor’s Manual
Free learning resource on English grammar, punctuation, usage, and style.
BCE and CE are religiously neutral alternatives to BC and AD. While BC, BCE, and CE appear after the date (223 BC), AD appears before (AD 1776). Periods are optional and generally omitted.
Capitalize specific time periods (Middle Ages), but lowercase descriptive terms (medieval times). Also capitalize proper names (Victorian era) and geological time periods (the Jurassic).
Don’t enclose the abbreviation for a time zone in commas, and don’t use periods after the letters of the abbreviation. Full forms may or may not be capitalized. With an international audience, UTC offsets are often preferred.
Simply use the words “noon” and “midnight” instead of a.m. and p.m. On the 24-hour clock, both 00:00 and 24:00 indicate midnight. In informal usage, “12 o’clock” can be either noon or midnight.
The term “o’clock” indicates the hour and is a contraction of the phrase “of the clock.” Use either numerals or words (ten o’clock or 10 o’clock). “A quarter to,” “a quarter past,” and “half past” show times 15 or 30 minutes before or after the hour.
On the 24-hour clock, the day is divided into 24 hours designated by the numerals 00 for the first hour and 23 for the last hour. Both hours and minutes are always shown. A colon acts as separator between hours and minutes (16:45 for 4:45 p.m.).
Use numerals or figures to write time with “a.m.” and “p.m.” (6:30 p.m.) and in the 24-hour system (18:30). With “o’clock,” use either numerals or words (2 o’clock or two o’clock). Use words for quarter, half, and whole hours (half past ten).
Periods are used when the abbreviations for time are lowercased (a.m., p.m.) but omitted when they are capitalized (AM, PM).
Both the hour and the minute are usually shown with “a.m.” and “p.m.” (e.g., 8:15 p.m.). But for whole hours, simply indicating the hour suffices (8 a.m. or 8:00 a.m.). Stay consistent in usage within a single context.
The abbreviations “a.m.” and “p.m.” are generally lowercased in running text, though they may also be capitalized. When lowercased, the letters are followed by periods; when capitalized, periods are omitted.