Manual of style/Time and date

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Times

Context determines whether the 12-hour clock or 24-hour clock is used; in both, colons separate hours, minutes, and seconds (1:38:09 pm and 13:38:09).

• 12-hour clock times should end with dotted lower-case a.m. or p.m., which are spaced (2:30 p.m., not 2:30p.m.). A hard space is advisable: 2:30 pm. Use noon and midnight rather than 12 p.m. and 12 a.m.; whether midnight refers to the start or the end of a date will need to be specified unless this is clear from the context.
• 24-hour clock times have no a.m., p.m., noon, or midnight suffix. Use discretion as to whether the hour has a leading zero (08:15 or 8:15). 00:00 refers to midnight at the start of a date, 12:00 to noon, and 24:00 to midnight at the end of a date.

Longer periods

• Write months as whole words (February, not 2), except in the YYYY-MM-DD format (as in 2000-04-01). Use abbreviations such as Feb. only where space is extremely limited, such as in tables and breakout boxes. Do not insert of between a month and a year (April 2000, not April of 2000).
• Seasons as dates. As the seasons are reversed in the northern and southern hemispheres—and areas near the equator tend to have just wet and dry seasons—neutral wording (in early 1990, in the second quarter of 2003, around September) is usually preferable to a "seasonal" reference (summer 1918, spring 1995). Even when the season reference is unambiguous (for instance when a particular location is clearly involved) a date or month may be preferable to a season name, unless there is a logical connection (the autumn harvest). Season names are preferable, however, when they refer to a phase of the natural yearly cycle (migration to higher latitudes typically starts in mid-spring).
• Season names are not normally capitalized.
• Years
  • Years are normally expressed in digits; a comma is not used in four-digit years (1988, not 1,988).
  • A slash may be used to indicate regular defined yearly periods that do not coincide with calendar years (the financial year 1993/94).
• Decades contain no apostrophe (the 1980s, not the 1980's); the two-digit form is used only where the century is clear (the '80s or the 80s).