time of day, elapsed time

(A similar topic is discussed in French in the article HEURE (ÉCRITURE DE L’HEURE).)

In references to time, follow the guidelines below.

Time of day

Except in descriptive text, write the exact time of day in numerical form. A non-breaking space should be placed between the time and the abbreviation a.m. or p.m. In Canada, a.m. and p.m. are usually written in lower case, with periods and no internal spacing:

• Does the bus arrive at 7:30 p.m. or 8:00 p.m.?
• Louis is picking me up at 11 a.m.

In approximations and descriptive text, write out whole, half and quarter hours in words, omitting the abbreviations a.m. and p.m. (which are used only with numerals):

• Drop by at around six o’clock for dinner.
• We arrived shortly after half past seven (or seven-thirty) in the evening.
• Time passed excruciatingly slowly—one o’clock, two o’clock, three o’clock …

When writing out the time of day in words, use a hyphen between the hour and the minutes, unless the minutes themselves are hyphenated:

• I leave for work between eight and eight-thirty.
• Megan usually leaves for work at about eight forty-five.

The 24-hour system

The 24-hour system is used in specialized fields (such as the military) and in French Canada; it is also the most commonly used system internationally.

Therefore, in documents presented in both official languages, and in all forms of international communication, it may be desirable to use the 24-hour system for representing time of day, in accordance with International Standard ISO 8601 and the Treasury Board Federal Identity Program Manual.

In this system, the abbreviations a.m. and p.m. are not used. The hour is represented by a two-digit number ranging from 00 to 23 (or 24), while the minutes are represented by a two-digit number ranging from 00 to 59. The colon is used as a separator between hour and minute:

• 00:15 (12:15 a.m., i.e. 15 minutes after midnight)
• 08:00 (8:00 a.m.)
• 12:00 (noon)
• 15:39 (3:39 p.m.)
• 24:00 (midnight)

Seconds may also be represented by a two-digit number ranging from 00 to 59, with a colon between minute and second:

• 14:12:26 (12 minutes and 26 seconds after 2:00 p.m.)

To convert time from the 12-hour to the 24-hour system, simply add 12 hours to any hour after noon:

To avoid confusion between 12 a.m. and 12 p.m., use the 24-hour system, or write out the time in words, depending on the context:

• For 12 a.m., use 00:00 (or 24:00), or midnight.
• For 12 p.m., use 12:00 or noon.

Spans of time

Use an en dash to link two figures that represent a continuous sequence. Do not put a space on either side of the en dash:

• Business hours: 9:00 a.m.–5:00 p.m. (or 9 a.m.–5 p.m.)

Do not use the en dash if the sequence is introduced by from or between. From must be followed by to, and between by and:

• He was absent from 9 to 10 a.m. (not from 9–10 a.m.)
• Lunch is served between 11:30 a.m. and 2:00 p.m. (not between 11:30–2:00 p.m. and not between 11:30 to 2:00 p.m.)

Elapsed time

For elapsed time, use colons, periods and no spaces:

• 2:30:21.65 (hours, minutes, seconds, tenths, hundredths)

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

These symbols should also be used when units of time are expressed with SI units:

• 16 km/d (16 kilometres per day)
• 10 J/h (10 joules per hour)
• 18 m/s (18 metres per second)
• 60 r/min (60 revolutions per minute)

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