Whether you’re a student, writer, or teacher, time is something that always seems in unfortunately short supply. Questions about how to write about time, though, are abundant. You may be clear now on whether you should use long form instead of numerals, but what about the 24-hour clock? Time zones? Let’s spend a little time (ahem!) sorting it out:
Hours and Minutes
Unless you’re using the 24-hour clock, separate the hours and minutes with a colon. For times that are right on the hour, Chicago style uses the colon plus 00; AP style does not (and omits the colon in that case). Canadian style has no distinct preference, so check with your style guide first; otherwise, stay consistent.
Example A (Chicago style): The flight arrives at 11:00 p.m.
Example B (AP style): The flight arrives at 11 p.m.
Remember that regardless of what style you use, it’s noon and midnight rather than 12 a.m. or 12 p.m.
24 Hours A Day
In North America, the 24-hour clock is primarily used in the military, in medical settings, or in some government communication. It is always written in numerals, with or without a colon separating hours and minutes: for example, 1300, 13:00, or 1300h or 13:00h (the h stands for hours). Note that unlike with a.m. and p.m., there’s no space between the time and h.
Whether or not to use a colon depends on your audience. Scientific convention uses a colon, as does the federal government. But if you’re writing about military time in a North American context, omit the colon.
Either way, if it’s a single-digit hour, use a zero for the other digit. And don’t ever use a.m. and p.m. with the 24-hour clock.
Example C (military context): The shift will commence at 0330.
Example D (federal government context): The meeting will commence at 13:30.
Sometimes including the time zone is necessary for clarity, particularly if you’re writing for an audience spanning several time zones. There are just a few short rules: Time zones are abbreviated using capital letters, with no periods. Chicago style puts them in parentheses; AP style does not. And there’s no need to spell them out.
Example E (Chicago style): The conference will be held at 11:30 a.m. (MST).
Example F (AP style): The conference will be held at 11:30 a.m. MST.
A caution: if you’re writing about a location that is on daylight saving time?or that isn’t when most other locations are?then be sure to specify that. 4:30 p.m. EST (Eastern Standard Time) and 4:30 p.m. EDT (Eastern Daylight Time) mean two different things.
In next week’s issue, we’ll finish this series on time of day with a look at a.m. and p.m.: when and how to use them, and most importantly, how to punctuate them. In case you can’t wait, though, here’s a hint: use lowercase with periods. More details next week.
Christina M. Frey is a book editor, literary coach, and lover of great writing. For more tips and techniques for your toolbox, follow her on Twitter (@turntopage2) or visit her blog.