FAQs on Writing the Time of Day


Ce contenu est offert en anglais seulement.

Barbara Collishaw
(Terminology Update, Volume 35, Number 3, 2002, page 11)

Frequently asked questions: Should it be a.m. (Ante Meridiem) and p.m. (Post Meridiem) or A.M. (Ante Meridiem) and P.M. (Post Meridiem)? Are there always periods in those abbreviations? Is it precise or redundant to write "4:30 o’clock p.m. in the afternoon"? When is it appropriate to use the twenty-four hour clock? Is there such a thing as "metric time"? How can the time be expressed in numbers alone? How many time zones does Canada have? What does UTC stand for? Are there rules and standards or is it all a matter of personal taste and convenience?

The answers vary according to context and sources consulted; there is more agreement, however, about times than dates.

Capitalization and periods

In the traditional 12-hour clock system, The Canadian Style advises that a.m. (Ante Meridiem) and p.m. (Post Meridiem) should be written with periods, in lower case (3:30 a.m. (Ante Meridiem), 3:30 p.m. (Post Meridiem)). Such Canadian reference works as The Globe and Mail Style Book, The Canadian Press Style Book, The Gregg Reference Manual, the Gage Canadian Dictionary and The Canadian Oxford Dictionary agree.

There are many other possible styles, and one of them may prevail in your place of work or among your correspondents. The usual alternative forms are regular or small capitals (A.M. (Ante Meridiem), P.M. (Post Meridiem), A.M. (Ante Meridiem), P.M. (Post Meridiem)). The Nelson Canadian Dictionary’s preference goes to capitals over lower-case letters or small capitals, while the Oxford Guide to Canadian English Usage says all these forms are correct. In headlines, lists and tables written in capital letters, a.m. (Ante Meridiem) and p.m. (Post Meridiem) are also capitalized. The experts agree: periods are always used, with no internal spacing.

Except in descriptive text and in approximations, write the time of day in numerical form:

The program will be broadcast at 8:05 p.m. (Post Meridiem)


He said that he would call after ten o’clock.

In a scientific or technical context, express precise measurements of elapsed time by means of the internationally recognized symbols of time d for day, h for hour, min for minute and s for second:

7 h 20 min flying time
The test run took 1 d 3 h 43 min 09 s precisely.

Source: The Canadian Style, 1997, Section 5.12

Precision or redundancy?

The phrase "4:30 o’clock p.m. (Post Meridiem) in the afternoon" may be criticized from several angles. First, it is redundant to write or say both p.m. (Post Meridiem) and in the afternoon, since they mean the same. Similarly, both o’clock and p.m. (Post Meridiem) are indications of time, and so only one is required. O’clock is best suited to express approximate times, rather than precise ones, and is often linked with times written out in words. Consider these options: four o’clock, 4 o’clock, 4:30 p.m. (Post Meridiem), 4:30 in the afternoon, but not 4:30 o’clock.

24-hour clock

Although everyday prose and speech retain the 12-hour system, the 24-hour clock is preferable for greater precision and maximum comprehension the world over. Transportation schedules, radio communications and computer-data transmission are clearer and more succinct when times are expressed as 13:40 rather than 1:40 p.m. (Post Meridiem), for example. With the 24-hour clock, there is no more confusion between 12 a.m. (Ante Meridiem) and 12 p.m. (Post Meridiem) Note that it is redundant to write 12 noon or 12 midnight; 12 is unnecessary.

Term 12-hour system 24-hour clock
midnight 12 a.m. 24:00 or 00:00 (see box)
noon 12 p.m. 12:00


What about the metric system (SI)?


The second is an official SI (International System) unit, while hour and day are not strictly SI, but are accepted or "customary" units that are used with the SI. The symbol for second is s, while min and h stand for minute and hour. These are not abbreviations, but symbols; there is no period and no s to indicate a plural. There is one space between the number and the symbol (e.g. The broadcast lasted 15 min 30 s and was heard everywhere).

Representation of time of day

The hour is represented by a two-digit number ranging from 00 up to 23 (or 24), the minute and second are represented by a two-digit number ranging from 00 up to 59, and the colon is used as a separator between hour and minute and between minute and second, as illustrated below:

24-hour representation
with seconds without seconds
00:15:00 00:15
08:00:00 08:00
12:00:00 12:00 (noon)
24:00:00 24:00 (midnight)
07:15:00 07:15
11:37:00 11:37
14:12:26 14:12

The instant of midnight should be represented (when seconds are included) as either 24:00:00, the end of one day, or 00:00:00, the beginning of the next day, according to circumstances.

Source: The Canadian Style, Section 5.13

Numbers only

For times written entirely in numbers, the Standards Council of Canada encourages the use of the National Standard of Canada (CAN/CSA-Z234-4), which is essentially the same as standard ISO 8601:2000, issued by the International Organization for Standardization, covering the presentation of dates and times for information exchanges. According to these standards, both dates and times are written in decreasing order of magnitude from left to right. Times are formatted HHMMSS (basic format) or HH:MM:SS (extended format), where H represents the two digits of the hours, M the minutes and S the seconds; a leading zero is always added to single-digit numbers. The basic format (without colons) is appropriate when computer readability and storage space are of primary importance and the extended format (with colons) when the document is intended for general readers. The 24-hour clock is always used.

According to the ISO standard, July 1, 2002, is written 2002-07-01. If the Canada Day fireworks were set to begin at 10:30 that evening, the computer that controls them would be set for 22:30:00. The entire date and time would be written as follows (T indicates the beginning of the time notation):

2002-07-01T22:30:00 or 20020701T223000

Time zones

Of course, those Canada Day fireworks on Parliament Hill would be scheduled for 22:30:00 EDT (eastern daylight time), while the fireworks in Vancouver might be scheduled for 22:30:00 PDT (Pacific daylight time). Some revellers in Ottawa may have gone home to bed by the time Vancouverites are celebrating, for by then it will be 01:30:00 EDT. Since Canada covers six time zones, it is necessary to specify which time zone is the reference point, when co-ordinating nationwide activities.

The correct name for the civil (legal or official) time prevailing in most of North America from April to October is daylight saving time, not savings; on other continents, this time shift is also known as summer time. The civil time during winter months is standard time. When the name of the time zone is included, daylight saving time is shortened to daylight time with a corresponding abbreviation, e.g. Pacific daylight time or PDT. Time zones may be stated with either the 12-hour or 24-hour systems (3:15 a.m. ADT or 03:15 ADT). When written out in full, Canada’s time zones appear in lower case (e.g. mountain daylight time) except the words Atlantic, Pacific and Newfoundland, which are normally capitalized. All abbreviations are in upper case and have no periods:

Canada’s Time Zones
Winter time zones At 9 a.m. (09:00) in Ottawa Summer time zones
  abbr. relative to UTCRemark a   abbr. relative to UTC
Pacific standard time PST -8 h 06:00 Pacific daylight time PDT -7 h
mountain standard time MST -7 h 07:00 mountain daylight time MDT -6 h
central standard time CST -6 h 08:00 central daylight time CDT -5 h
eastern standard time EST -5 h 09:00 eastern daylight time EDT -4 h
Atlantic standard time AST -4 h 10:00 Atlantic daylight time ADT -3 h
Newfoundland standard time NST -3 h 30 min 10:30 Newfoundland daylight time NDT -2 h 30 min



The reference point for the world’s time zones is the "prime meridian," which passes through the Royal Observatory Greenwich (known until 1998 as the Royal Greenwich Observatory) near London, England. The name "Greenwich Mean Time" was discontinued in 1972 and Coordinated Universal Time is its modern successor. UTC (Universal Time Coordinated) was selected as the symbol in order to be language-neutral, rather than CUT in English and different forms in other languages.


  1. The Canadian Style, 1997, Sections 1.21, 1.22, 5.12 and 5.13.
  2. International Standard ISO 8601, Second Edition, 2000-12-15. Reference number: ISO 8601:2000(E).
  3. A Summary of the International Standard Date and Time Notation by Markus Kuhn at http://www.cl.cam.ac.uk/~mgk25/iso-time.html.
  4. The National Research Council of Canada Web site at http://www.nrc.ca/inms/inmse.html.
  5. "Dated and Confused?" article on Standards Council of Canada Web site at http://www.scc.ca/news/articles/dc-e.html.
  6. The Globe and Mail Style Book, McClelland & Stewart Inc., 1998.
  7. The Canadian Press Style Book, The Canadian Press, 1999.
  8. The Gregg Reference Manual, McGraw-Hill Ryerson, 1999.
  9. Gage Canadian Dictionary, Gage, 1998.
  10. The Canadian Oxford Dictionary, Oxford University Press, 1998.
  11. Nelson Canadian Dictionary, ITP Nelson, 1997.
  12. Oxford Guide to Canadian English Usage, Oxford University Press, 1997.
  13. Federal Identity Program Manual, 1990, pages 26 and 27.
  14. The International System of Units (SI), Seventh Edition, Bureau international des poids et mesures, 1998.

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