You take it out of the box and plug it in and it sets itself. Amazing.
Magic. Assisted by one of the several national atomic clock time signals.
It recieves radio wave transmissions.
That is one way, but thisclock which I bought just has the time set at the factory. A battery saves the time during power interruption (and shipping). You just have to set the local time zone. It even automatically adjusts for daylight savings time. I had an atomic clock once and it did not work in most places in the house becuase we were behind a hill.
If this is one from Emerson Research, I have the same type. It does not receive the WWV time broadcasts. Instead, it has what is basically a digital watch keeping time internally. It has a battery that is supposed to be good for 5 years, which may be recharged by the house current when it is plugged in. And when it is plugged in, you will see the clock update the year, month, day, hour and minute in sequence. You may have to set up your time zone and it does not keep perfect time, it will need to be adjusted occasionally.
that is the one i have.
Just a warning: these clocks are not terribly smart about daylight savings. I had three purchased before the DST schedule was changed three years ago or so… and the clocks keep adjusting it on the old schedule that was pre-programmed into them.
I have a wristwatch that resets itself to the right time at 3 am every day, assuming it can get the radio signal. It runs normally the rest of the time. This was fine until I went to Europe, where it would go off line in the middle of the afternoon. 
I don’t know how often standard atomic clocks reset. Are they always checking or only once in a while, like my watch?
I think the atomic clocks need to be reset every few hundred thousand years (depending on the particular model). 
I have a clock that uses the WWV signal from the atomic clock in Boulder, Colorado, and it checks every hour. If it’s somewhere that it can’t get a signal, it just continues to operate without updating. It’s accurate enough on its own that updating hourly (or even daily) is overkill.
Don’t some FM radio stations include the time in their signal? Trying to remember how that works. I know that my car displays the call letters of each station I have tuned in, so some data must be included in the signal.
A work around (if you haven’t already figured it out) for a clock that doesn’t follow the revised DST schedule is to configure the clock not to use DST (this should be an option because parts of the US don’t follow DST), then adjust the time zone yourself when you enter and leave DST.
How does it know what time zone it’s in?
Here in Japan, we have a radio-controlled alarm clock with two separate alarm settings, one for me and one for my better half. It cost US$20 about 3 years ago and we just recently changed the battery.
There should be a time zone button or setting just like there is a daylight saving time zone setting. The time standard radio station broadcasts in a standard time zone and clocks have to be adjusted accordingly by their users so they display the proper local time.
Arthur C. Clarke is often quoted as saying, “Any sufficiently advanced technology is indistinguishable from magic,” but my personal corollary to that is “Any sufficiently mature technology is as simple as a flashlight.”
Stranger
I have one of those, too. I plugged it in and it was four minutes off. There’s no way to reset it and it drives me nuts.
Well, you’re just gonna have to move one meridian east. Or west.
I don’t think U.S. FM stations transmit the time, but the new digital TV stations do – just not very well. Few of the stations in the Raleigh-Durham, NC seem to have anyone making sure the time is correct, so the times are surprisingly inaccurate – especially since the morning shows all show accurate time on their standard programming – with many a few minutes off, and some up to 20 minutes off.
I deserved that - especially because I went to a conference once at that site.
I suspect the reason my watch doesn’t do it hourly is power savings. It appears to go into a special mode to listen for the radio signals, and no doubt turns up the amplification on the receiver. It shuts off the display while looking. It is also solar powered, and so probably has some good power savings software built in.
[slight hijack with more than you probably wanted to know]
Television stations have had a mechanism for encoding time of day for well over a decade. In the analog NTSC signal it’s encoded in the VBI (specifically field 2 of line 21) along with a set of other information collectively called XDS (extended data services).
This is how VCRs have been able to set their own time for years now. If you have any other questions, ask away. I worked on the standards committee that defined XDS and wrote about it in one of my books.
[end of hijack]