When you tap World Clock on the Clock screen, you start out with only one clock, showing the current time in Apple's own Cupertino, California.

Sure, this clock shows the current time, but your phone's status bar does that. The neat part is that you can open up several of these clocks, and set each one up to show the time in a different city. The result looks like the row of clocks in a hotel lobby, making you look Swiss and precise.

By checking these clocks, you'll know what time it is in some remote city, so you don't wind up waking somebody up at what turns out to be three in the morning.

To specify which city's time appears on the clock, tap the button at the upper-right corner. The keyboard pops up so you can type the name of a major city. As you type, a scrolling list of matching city names appears above the keyboard; tap the one whose time you want to track.

As soon as you tap a city name, you return to the World Clock display. The color of the clock indicates whether it's daytime (white) or night (black). Note, too, that you can scroll the list of clocks. You're not limited to four, although only four fit the screen at once.

To edit the list of clocks, tap Edit. Now you can delete a city clock by tapping and then Delete, or drag clocks up or down in the list using the grip strip as a handle. Then tap Done.

If you travel much, this feature could turn out to be one of your iPhone's most useful functions. It's reliable, it's programmable, and it even wakes up the phone first, if necessary, to wake you.

To set an alarm, tap Alarm at the bottom of the Clock program's screen. Tap the button at the upper-right corner to open the Add Alarm screen.

Your options here are:

When you finally tap Save, you return to the Alarm screen, which lists your new alarm. Just tap the On/Off switch to cancel an alarm. It stays in the list, though, so you can quickly reactivate it another day, without having to redo the whole thing. You can tap the button to set another alarm, if you like.

Note, too that the icon appears in the status bar at the top of the iPhone screen. That's your indicator that the alarm is set.

To delete or edit an alarm, tap Edit. Tap and then Delete to get rid of an alarm completely, or tap the alarm's name to return to the setup screen, where you can make changes to the time, name, sound, and so on.

So what happens when the alarm goes off? The iPhone wakes itself up, if it was asleep. A message appears on the screen, identifying the alarm and the time.

And, of course, the sound rings. This alarm is the one and only iPhone sound that you'll hear even if the silencer switch is turned on. Apple figures that if you've gone to the trouble of setting an alarm, you probably really want to know about it, even if you forgot to turn the ringer back on.

In that case, the screen says, "slide to stop alarm."

To cut the ringing short, tap OK or Snooze. After the alarm plays (or you cut it short), its On/Off switch goes to Off (on the Alarms screen).

You've never met a more beautiful stopwatch than this one.

Tap Start to begin timing something: a runner, a public speaker, a train, a long-winded person who's arguing with you.

While the digits are flying by, you can tap Lap as often as you like. Each time, the list in the bottom part of the screen identifies how much time had elapsed at the moment you hit Lap. It's a way for you to compare, for example, how much time a runner is spending on each lap around a track.

You can do other things on the iPhone while the stopwatch is counting, by the way. In fact, the timer keeps ticking away even when the iPhone is asleep! As a result, you can time long-term events, like how long it takes an ice sculpture to melt, the time it takes for a bean seed to sprout, or the length of a Michael Bay movie.

Tap Stop to freeze the counter; tap Start to resume the timing. If you tap Reset, you reset the counter to zero and erase all the lap times.

The fourth Clock mini-app is a countdown timer. You input a starting time, and it counts down to zero.

Countdown timers are everywhere in life. They measure the time of periods in sports and games, of cooking times in the kitchen, of stunts on Survivor. But on the iPhone, the timer has a especially handy function: It can turn off the music or video after a specified amount of time. In short, it's a Sleep timer that plays you to sleep, then shuts off to save power.

To set the timer, open the Clock app, and then tap Timer. Spin the two dials to specify the number of hours and minutes you want to count down.

Then tap the When Timer Ends control to set up what happens when the timer reaches 0:00. Most of the options here are ringtone sounds, so you'll have an audible cue that the time's up. The top one, though, Sleep iPod, is the aforementioned Sleep timer. It stops audio and video playback at the appointed time, so that you (and the iPhone) can go to sleep. Tap Set.

Finally, tap Start. Big clock digits count down toward zero. While it's in progress, you can do other things on the iPhone, change the When Timer Ends settings, or just hit Cancel to forget the whole thing.