# How accurate is the time on a digital clock versus the time on internet clocks? [closed]

How accurate is the time on a digital clock versus the time on internet clocks? The internet clocks seem slower than internet clocks. So which is the correct time? What are the most accurate clocks?

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## closed as off-topic by dmckee♦Feb 5 '14 at 17:47

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You have a problem with your question, in that you suggest one set is slower than itself. More over, I am not sure what makes ths question suitable for Physics.SE? – dmckee Aug 10 '11 at 17:26

How about the US Naval Observatory site? I think they give master clock signals (the ones you would get over an "atomic" radio-controlled clock that comes from the cesium clock they use, which is the same one (I think) that the beep signals used to come from. I have an inexpensive radio-controlled clock because I always like to know how much time goes by while I'm doing something that would be better spent doing something else. A battery operated clock works during power outages, if you remember to replace the battery once a year or so.

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Alas the USNO tick and tock servers are no longer publicly available, and the observatory suggests that you use a tier two server near you. – dmckee Jul 20 '15 at 1:20

There are two parts to the question - how regular are the clocks (is a second always the same) and how close is the reading on the clock to the official time now.

A good digital clock, with reasonably constant temperature is probably good to 1sec/day. That is a second has the same value to within 1/(24*60*60) = 0.001%

An atomic clock can be accurate to 1second in millions of years. Fortunately there are 24 expensive atomic clocks flying over your head broadcasting the time to your GPS receiver. Most internet service providers and cell phone masts also use GPS as the time reference so the time on your cell phone and your computer (if you set auto time sync) are very 'accurate'

Then there is the problem of the delay between a distant clock knowing that it it '12:00' and getting the message to you. The GPS receiver is the best - the travel time for the signal from the satelite to you is a few millisecs. For your computer it may take 10s of millisecs for the time message to go over the network and so the system has tricks to try and estimate this and correct for it.

Radio used to be the standard for time signals. At the start of the news the radio would play a series of beeps (the pips) that accurately gave you the start of the hour. Now that the radio signal is digitised, sent over a computer network to the transmitter, recorded by the computer in your radio, decompressed, and then played - there may be a delay of several seconds, so most broadcasters no longer send the time signal

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The clocks that you find on websites are delayed a bit between the time that the time is registered until it gets to you. Also, you cannot know if the server time is set properly.

There does exists a timeserver protocol, but a human will not make sense of it. Your best bet is to set your computer clock to sync to a timeserver, then reference your computer clock.

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NTP has provisions to measure and correct for this. – dmckee Aug 11 '11 at 3:01