11 Numbers and dates
11.3 Times of day
The formulation of times of day is a matter of editorial style, and different forms are more or less appropriate to particular contexts. It is customary to use words, and no hyphens, with reference to whole hours and to fractions of an hour:
half past four
a quarter to four
Use o’clock only with the exact hour, and with time expressed in words: four o’clock, not half past four o’clock or 4 o’clock. Do not use o’clock with a.m. or p.m., but rather write, for example, eight o’clock in the morning. Use figures with a.m. or p.m.: 4 p.m. Correctly, 12 a.m. is midnight and 12 p.m. is noon; but since this is not always understood, it may be necessary to use the explicit 12 midnight and 12 noon. The twenty-four-hour clock avoids the use of a.m. and p.m.: 12.00 is noon, 24.00 is midnight. In British English either a colon or a full point as a separator is acceptable if applied consistently.
In the twelve-hour clock, use figures when minutes are to be included: 4.30 p.m. For a round hour it is not necessary to include a decimal point and two zeros: prefer 4 p.m. to 4.00 p.m. In North America, Scandinavia, and elsewhere the full point is replaced by a colon: 4:30 p.m. This is often seen in British usage too. Some styles omit the punctuation in a.m. and p.m.