The Editor’s Manual
Free learning resource on English grammar, punctuation, usage, and style.
Use “a.m.” and “p.m.” to indicate exact time, and use numerals instead of words to denote the hour and the minute. These abbreviations are generally lowercased in running text, but they may also be capitalized.
When exact time is important, use numerals with “a.m.” or “p.m.” (“2:33 p.m.”); otherwise, use words (“two o’clock,” “two thirty,” “half past two”). In certain settings, the 24-hour system is used to show exact time (“14:33” or “1433 hours”).
Spell out single-digit numbers from zero to nine and larger numbers at the start of a sentence. Use numerals with abbreviated units of measurement, to emphasize the exact time, and to write the names of years.