Measures of Time
19.1.3 Measures of Time
Measures of time usually are expressed as numerals (see also 14.3, Abbreviations, Days of the Week, Months, Eras). When dates are provided, numerals should be used for day and year; the month should be spelled out unless listed in a table. Conventional form for time and dates (11:30 pm on February 25, 1961) is preferred to European or military form (2330 on 25 February 1961). However, use of military time may clarify the time course in figures that depict a 24-hour experiment, times of drug dosing, and the like. For time, if the hour of the day is given, am or pm is used and set in small capitals (see also 22.0, Typography). When referring to time on the hour, the minutes may be omitted (eg, 3 pm). With 12 o'clock, simply use noon or midnight, whichever is intended.
At 5:45 am, October 15, 1994, the researchers completed the final experiment.
The 21st century officially began just after midnight on January 1, 2001.
When referring to a position as it would appear on a clock face, express the position by means of numerals followed by “o'clock.”
The needle was inserted at the 9-o'clock position.
But: The procedure was scheduled to begin at 9 am.
See 8.2.3 (Punctuation, Comma, Semicolon, Colon, Colon) for punctuation in expressions of time.