英语语法：121 Daylight-Saving Time

（单词翻译）

by Mignon Fogarty

Many parts of the world are moving from standard time to daylight-saving time (also called summer time) this week, so I thought it would be a good time to talk about the phrase “daylight-saving time” and time in general.
The original phrase was "daylight-saving time," and it is still generally agreed to be "saving," not "savings," time (1, 2). Remember the name by thinking that you are saving light, daylight, to be exact. The words are not capitalized and whether to use a hyphen between "daylight" and "saving" is a style choice.* I prefer the hyphen because I think of "daylight-saving" as a compound modifier that modifies "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 don't participate in daylight-saving time, and a few places divide their region into half-hour zones. Actually, it's even more complicated than that. Arizona, for example, doesn't participate in daylight-saving time, but other states in the same time zone do. So during standard time, it is the same time in Arizona and Utah, but during daylight-saving time, it is an hour earlier in Arizona because Arizonans don't "spring forward" like other regions in their time zone.
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 parentheses¹ after the time: 4:00 p.m. (EST) [for eastern standard time].
However, as many readers have noted², it's common for people not to know whether we're in daylight-saving time and to write EST throughout the year and not just during standard time. If you're one of those people, it's better to simply use "ET" as an abbreviation for "Eastern Time" rather than write it wrong.
Multiple international listeners have suggested using GMT (Greenwich Mean Time)  as an alternative because it is the same for everyone. Evan made a good point that other countries may use names such as "Eastern time" for their own time zones which could cause confusion when they are arranging meetings with people in the U.S. Eastern time zone. But I had to laugh when he wrote that "12:30 p.m. GMT-5 is completely unambiguous." Although it may be unambiguous, I would have to look up the conversion³ online, as would, I suspect, 95% of Americans. So although it may be unambiguous, it wouldn't be an improvement in ease of communication, at least not among the people I know. It may be good to use GMT if you regularly schedule meetings internationally, but I can't recommend it for people who mainly communicate with other people within the U.S.
However, it is polite to describe your meeting times in the other person's time zone. For example, I could say something like, "Let's talk at 9:00; that's noon your time." Just make sure you get the conversion right! I always use TimeAndDate.com to check on times in other cities, and I have a widget on my desktop⁴ that shows the time in the three different cities where my regular business associates work.
[Note: I couldn't find a convincing rule about capitalizing time zone names. The Chicago Manual of Style lists the full names in lowercase, with "Pacific" in "Pacific time zone" capitalized. The Associated Press Stylebook recommends capitalizing each word in the name when you write it out. The Writer's Guide Grammar Desk Reference shows "Eastern standard time" but "central standard time." All three guides use all caps when abbreviating⁵ 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," not "meridiaN⁶."
You can write "a.m." and "p.m." as lowercase letters with periods after them or as small capitals without periods (3, 4). 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.")(5). I suspect this is because it’s so hard to make small caps on a computer.
   
Rae asked whether it's OK to write 2 p.m. instead of 2:00 p.m.? I would consider it acceptable in informal situations such as a flyer promoting a pizza party, but in formal writing, you should write out the zeros (6).
Noon and Midnight
Remember how I said a.m. means "before noon" and p.m. means "after noon"? So what about noon, then? Technically⁷, 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" (7, 8, 9).
Period of Time
There are also a couple common redundancies that relate to time.
For example, it's redundant⁸ 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"--either "period" alone or "time" alone will suffice (10). Marc in Long Beach, California, wrote in about this recently: 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 use either "period" or "time" 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" between the two numbers: from 7:00 p.m. to 9:00 p.m. (14, 15).
Finally, in case you were wondering, in 2010 in the United States, daylight-saving time begins on Sunday, March 14, so set your clocks forward one hour before you go to bed on Saturday, March 13.
Sleep Phones

This is National Sleep Awareness⁹ Week, and I'm thrilled to have Sleep Phones as an advertiser because these little pajamas¹⁰ for your ears have dramatically improved my ability to sleep. It's a soft headband with flat speakers by your ears, so you can lie on your side or your back and listen to podcasts or audiobooks while you fall asleep. I'm practically addicted¹¹ to them; I even take them with me when I travel. Get a $5 discount with the code "GRAMMAR" at sleepphones.com/gg.

Grammar Girl's Quick and Dirty Tips for Better Writing

I'm Mignon Fogarty, author of the New York Times best-seller, Grammar Girl's Quick and Dirty Tips for Better Writing. Pick it up wherever fine books are sold. Hundreds of people have written in over the years telling me how helpful the book has been for them to have on hand, and if you enjoyed this article, you'll definitely enjoy the book.

That's all. Thanks for listening!