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Times of Day

Is it am or a.m.? When do I spell out hours? How do I use o’clock?

Three good questions. Here are the answers:

  1. Use a.m. and be consistent.
  2. Even, half, and quarter hours are usually spelled out in text. ( half past five, quarter to six)
  3. When you use o’clock, always spell out the number. ( Five o’clock, not 5 o’clock)

10.41Abbreviations for time of day

The following abbreviations are used in text and elsewhere to indicate time of day. Though these sometimes appear in small capitals (with or without periods), Chicago prefers the lowercase form, with periods, as being the most immediately intelligible. 

The abbreviations a.m. and p.m. should not be used with morningafternooneveningnight, or o’clock

All answers are based on the Chicago Manual of Style Online (CMOS 17) and the Merriam-Webster Online Dictionary.

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