What Time Is It?

A man with a clock knows what time it is. A man with two clocks is never sure. ”

ancient proverb

 

Many modern technologies depend on very accurate time keeping. Global navigation has always depended on accurate time keeping; navigating by the stars or by the sun’s position in the sky only works if you know what time it is. The more accurate your close, the more accurate your positional calculation. Train schedules in the nineteenth century only made sense if cities worked off of agreements about what time it was. If Chicago and Kansas City have noon being thirty minutes apart because it takes the sun that long to cover that much of the sky, what does it mean if the Chicago to KC train is due at 1 PM? Worse, if a track is serving traffic in both directions, with some convention saying when a train has to pull into a siding to let an oncoming train pass, those converging trains better agree what time it is if the convention depends on time.

For eighty years or more now, there have been radio signals broadcasting the time. Short wave receivers to hear or process the signal may not have been common, but if you had the need, the signal was there. Thirty years ago, the GPS constellation of satellites was built, and the GPS system depends on those satellites agreeing very precisely what time it is; a receiver processes a signal with both a timestamp and an origin location and does more math than I can do to figure out where it must be if those are the signals it just received. About twenty-five years ago, cell phones started to enter common use, and they used a technique similar to the GPS to figure out which cell tower was closest and also where it was, at least roughly.

Now that almost all general purpose computers in use are on networks, the common operating systems all will use a network time server to make sure their clock is reasonably close to everyone else’s. They don’t have to agree as precisely as GPS satellites do, but when you’re trying to debug client-server computer problems, it’s much, much easier if the client and the server agree within a second or less what time it is.

Now even nearly generic alarm clocks are decoding one of the time radio broadcasts to set themselves. The user sets the time zone and the DST setting; the clock will set the minutes and seconds on its own.

So, given all of that, why doesn’t my hotel room alarm clock agree with my cell phone about what time it is?

It’s not a matter of seconds; it’s roughly two minutes. And it’s not just one kind of alarm clock; my wife’s Sony clock radio from Christmas a couple of years ago has the same quirk; it runs a couple of minutes behind my cell phone. I just compared http://www.time.gov with my laptop and my cellphone; all agree within a second of each other what time it is.

I suppose a good Google search might resolve this, but at this point I’m enjoying the analysis more than I’ll enjoy the answer.

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