英语语法：41 Daylight-Saving Time（在线收听）

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 time in general.

Most usage guides recommend using a hyphen, 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 for the hyphen too -- 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 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 later in Arizona. Because of this confusion, I'm always having to explain time zones to business associates who are in other states.

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].

[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. 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 (1, 2). 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.)(3). I suspect this is because it’s so hard to make small caps on a computer.

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 (4, 5, 6).

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 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 (7, 8). Merriam-Webster's Dictionary of English Usage argues that occasionally including the words period of adds clarity (9), but I'm willing to bet that 19 times out of 20 you can leave them out without causing confusion.

Mobile Grammar Girl

I just discovered that you can get the Grammar Girl podcast on a Nokia cell phone. I don't have a Nokia phone, so I can't tell you how it works, but Nokia tells me they have a podcast portal for their cell phone users.

Administrative Stuff

You can find a transcript of this podcast, my contact information, and all the other great Quick and Dirty Tips podcasts at quickanddirtytips.com. I'm on the road and recorded this episode early, so there won't be a Facebook quiz or Behind the Grammar episode this week.

Finally, in case you were wondering, in 2007 in the United States, daylight-saving time ends on the first Sunday in November (10), so set your clocks back one hour before you go to bed on Saturday November 3.

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