Day, night, anytime: Time and date formats

November 18, 2012

Based on a writing tip I recently sent out to my (Australian) work colleagues.


In this week’s writing tip I deal with times and dates and how to write them so that their meaning is clear and unambiguous to all readers. I’ve taken most of this information from our style authority, the most recent Australian Style Manual: For authors, editors and printers (6th ed, John Wiley & Sons, 2002)

Bottom line:

  • 24-hour time: four numerals only, no punctuation, no spaces (e.g. 1712)
  • 12-hour time: include am or pm, separate hours and minutes with a full stop (e.g. 9.30 am), use noon and midnight instead of the ambiguous 12 am or 12 pm
  • Avoid bi as a time frequency – use other words to say what you mean
  • Write dates in this order: day (numeral) month (word) year (numeral), with no punctuation e.g. 4 September 2012

Time formats

The Style Manual (p172-173) has:

  • 24-hour time: examples: 1700, 2318 etc. (‘Four digits are always used, the first two showing the hours and the last two the minutes. Neither punctuation nor space is inserted. Where more precise times that include decimal fractions of seconds are being expressed, colons can be used as the separator [as recommended by ISO 8601:2000]. For example: 23:59:17.’)
  • 12-hour time: examples: 5 am, 9.00 am, 7 pm, 10.15 pm etc. (‘…present am and pm in lower case … The use of full stops between these abbreviated words [a.m. and p.m.] is declining and, because they are always preceded by a numeral, they can be treated like other symbols associated with numerals, which are unpunctuated. A full stop should be used to separate the hours from the minutes. Two zeros may be used to indicate even hours but are not essential [i.e. 5 am or 5.00 am are both correct].’
  • The special case of noon and midnight: ‘Under the twelve-hour system, practice differs on the presentation of noon and midnight. Where confusion could be caused by using 12 am or 12 pm, it is preferable to use the terms noon and midnight. Thereafter, 12.01 pm refers to the beginning of the afternoon, and 12.01 am to the early morning.’

Date formats

The Style Manual (p170-171) has:

‘… dates are best presented using numerals for the day and year but with the name of the month … in full.’ Thus: 4 September 2012 (‘This structure is unambiguous, requires no punctuation, and progresses logically from day to month to year [...] and requires fewer keystrokes.’)

Unacceptable date formats include:

  • the 4th of September, 2012
  • 4th September, 2012
  • September 4th, 2012
  • September 4, 2012
  • 4 Sep 12 (unless space is very limited, such as in a table cell)
  • 4/9/2012 (this format is the LEAST ACCEPTABLE as it will be read by US readers as 9 April 2012. The Style Manual [p171] says: ‘All-numeral forms of dates can mislead because international practice varies, so ISO 8601:2000 should be followed in documents for wide distribution.’ [Note: ISO 8601:2000 specifies writing all-numeral dates in YYYYMMDD format, so 4 September 2012 would be 20120904; see also: http://cybertext.wordpress.com/2008/01/28/date-and-time-formats/]),

Words related to time

The special case of ‘bi’ words (Style Manual, p173): ‘Most prefixes that can be attached to an expression of time to indicate frequency are unambiguous—for example, tercentenary and triennial. But the prefix ‘bi’ poses problems, because it means both ‘two’ and ‘twice a …’; so bimonthly can mean either ‘every two months’ or ‘twice a month’. It is better to use more specific alternatives in place of this prefix, such as ‘twice weekly’ or ‘fortnightly’, or ‘twice monthly’ or ‘every two months’. In contrast, biannual and biennial each have one meaning only: respectively, ‘twice a year’ and ‘every two years’. Nevertheless they are often misunderstood, and so should only be used in contexts where their meaning can be made clear.’

And just to keep you on your toes, other time-related words are treated in various—sometimes conflicting—ways in the Macquarie Dictionary. For example:

  • afternoon and daytime (no hyphens/spaces) compared to night-time (hyphenated)
  • night shift and day shift (both separated by a space)
  • midnight, midday, and midafternoon (no hyphens/spaces)

[Link last checked November 2012]

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