Grammar Girl here.

Many parts of the world are moving from daylight-saving time (also called summer time) to standard time this week, so I thought it would be a good time to talk about the phrase “daylight-saving time” and dates and time in general.

But first, it's free audiobook time again because we're sponsored by Audible, the Internet's leading provider of spoken-word entertainment. Audible now has over 40,000 titles for you to choose from. Visit audiblepodcast.com/gg for a free trial that includes a free audiobook download today.

Most usage guides that address the question of how to write "daylight-saving time" directly recommend using a hyphen (1, 2) [Note: 3  and 4 do not use the hyphen, but also do not address the hyphen question directly], so the phrase is daylight[hyphen]saving time with no capital letters. Also it is saving, not savings, time. Just remember that you are saving time, daylight time, to be exact. That's the reason I side with the style guides that call for a hyphen -- daylight-saving is a compound modifier that applies to time.

Time Zones

Most countries have signed on to the idea of a standard world time system. For them the world is divided into 24 time zones, and each zone differs by an hour from the time zone next to it. Not everyone uses this system, though. Some time zones and regions within time zones don't participate in daylight-saving time, and a few places divide their region into half-hour zones.

If you need to indicate that a time is in a certain time zone, the simplest way to do it is to put the time zone abbreviation in parentheses after the time; for example, 4:00 p.m. (EST) [for eastern standard time]. I must caution you though that many people find time zones confusing and don't know when it's standard time and daylight time. So if it's important that people get the time right, I urge you to spell things out as much as possible. For example, if you're scheduling a meeting, you could write "We'll meet at 4:00 Pacific Time; that's 1:00 Eastern Time."

I couldn't find a consistent rule about capitalizing time zone names. The Chicago Manual of Style lists the full time zone names in lowercase, with the exception that Pacific in Pacific time zone capitalized. The AP Stylebook recommends capitalizing every word in the time zone name when you write it out. It seems to me that it is a style issue, so just choose a way of doing it and be consistent. Both guides use all caps when abbreviating the names (e.g., PST, EST).

AM and PM

Also, there are at least two acceptable ways to write a.m. and p.m., which are abbreviations for ante meridiem and post meridiem. Ante meridiem is Latin for "before noon" and post meridiem is Latin for "after noon." Note that it is meridieM, with an m, not meridiaN, with an n.

You can write a.m. and p.m. as lowercase letters with periods after them or as small capitals without periods (5, 6). Either way, there should be a space between the time and the a.m. or p.m. that follows. Although small capitals used to be preferred, it's now more common to see lowercase letters followed by periods (a.m. and p.m.)(7). I suspect this is because it’s so hard to make small caps on a computer.

In formal writing, you should still include the zeros after the colon when you are writing out an even hour. For example, 2:00 p.m. is written two colon zero zero p.m., not just the number two followed by p.m.

Noon and Midnight

Remember how I said a.m. means "before noon" and p.m. means "after noon"? So what about noon, then? Technically, noon is neither a.m. nor p.m. Although it's common to see noon written as 12:00 p.m. and midnight written as 12:00 a.m., it's not correct and can confuse people. It's better to stick with just the words noon and midnight (8, 9, 10).

Period of Time

There are also a couple common redundancies that relate to time.

For example, it's redundant to say "8:00 a.m. in the morning." By including the a.m. you've already indicated that it's morning.

It's also usually redundant to use the phrase "period of time." Marc in Long Beach, California, wrote in about this: He thought it would be better to say, "O.J. drove his van for a long time" than "O.J. drove his van for a long period of time." And Marc's right; there's no reason to say period of time when time will do just fine (11, 12). Merriam-Webster's Dictionary of English Usage argues that occasionally including the words period of adds clarity (13), but I'm willing to bet that 19 times out of 20 you can leave the words out without causing confusion.

And what about periods of time? How do you write that a party will start at 7:00 p.m. and end at 10:00 p.m.? Of course you could write it all out in a sentence as I just did, but if you want to shorten it, you use an en dash between the numbers and just one p.m. at the end: 7:00-9:00 p.m. But if you use the word from before the range of numbers, then you can't use a dash between them, you have to use the word to (t-o) between the two numbers. From and to--they go together (14, 15).

Administrative Stuff

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You can find a transcript of this podcast, my contact information, and all the other great Quick and Dirty Tips podcasts at quickanddirtytips.com.

Finally, in case you were wondering, in 2008 in the United States, daylight-saving time ends on the first Sunday in November (16), so set your clocks back one hour before you go to bed Saturday night, November 1.

My time's up, so that's all. Thanks for listening!

References

1. Goldstein, N., ed. The Associated Press Stylebook and Libel Manual. Reading: Perseus Books, 1998, p. 58.
2. Garner, B. Garner's Modern American Usage. Oxford: Oxford University Press, 2003, pp. 222-23.
3. "Daylight Savings Time" Brians, P. Common Errors in English Usage. http://www.wsu.edu/~brians/errors/daylight.html (accessed October 28, 2008).
4. Merriam-Webster's Dictionary of English Usage. Springfield: Merriam-Webster, 1994, p. 828.
5. "Time of Day," The Chicago Manual of Style. Chicago: The University of Chicago Press, 2006, section 15.44.
6. "Date and Time," The American Heritage Guide to Contemporary Usage and Style. Boston: Houghton Mifflin Company, 2005, p.131.
7. Garner, B. Garner's Modern English Usage. Oxford: Oxford University Press, 2003, p. 39.
8. Brians, P. Common Errors in English Usage. Wilsonville: William, James & Co., 2003, p. 10.
9. Garner, B. Garner's Modern American Usage. Oxford: Oxford University Press, 2003, p. 39.
10. Goldstein, N., ed. The Associated Press Stylebook and Libel Manual. Reading: Perseus Books, 1998, p. 208.
11. Garner, B. Garner's Modern American Usage. Oxford: Oxford University Press, 2003, p. 599.
12. Brians, P. Common Errors in English Usage. Wilsonville: William, James & Co., 2003, p. 205.
13. Merriam-Webster's Dictionary of English Usage. Springfield: Merriam-Webster, 1994, p. 728.
14. Ask the Editor, AP Stylebook Online http://www.apstylebook.com/ask_editor.php (accessed October 28, 2008).
15. Lutz, G. and Stevenson, D. Grammar Desk Reference. Writer's Digest Books: Cincinnati, 2005, p. 296.
16. Aldrich, B. "Saving Time, Saving Energy," California Energy Commission Web Site. http://www.energy.ca.gov/daylightsaving.html (accessed October 29, 2008).