10 ms·
I like Time.gov, personally; it was the first site I ever found around this concept and so it has a soft spot. I keep finding all my physical atomic clock sync
by mwexler 15y ago
I like Time.gov, personally; it was the first site I ever found around this concept and so it has a soft spot.
I keep finding all my physical atomic clock synced clocks (yes, I have more than one, they are cheap these days) disagreeing, sometimes by 2 seconds or more, which makes me laugh (great ideas ruined by poor implementation). I find many of the web sites (listed in comments or the original post) to also differ, perhaps for similar reasons of implementation choices.
I would presume all the sites work off various implementations of NTP, http://www.ntp.org/ http://www.ntp.org/ plus some trusted source.
I guess my question is: has anyone found a site which is really, really accurate by reducing all the latency and lag, so what you see on the screen really is, to whatever precision, accurate? And would said person have access to a really good source for the comparison point? I don't seem to have one. Yes, I should have stopped at 3 so that I could pick the 2 closest ones (like the old saying: 1 clock is unsure, 2 clocks are worse, but 3 at least lets you make a decision)
I wonder, would you need to have NTP on the client side synced to a trusted source (say, in java, flash, or javascript) to get a good reading? Any server serving over HTTP induces lag, I would think, and NTP is supposed to account for transmission delays, or so I recall.
Thanks for sharing, another interesting time site to add to the collection.
- ken 15y agoWhen I was a kid, my dad would always set our clocks by tuning the radio to one of the atomic clock broadcasts from Fort Collins. This might be the same source as your automatically synchronized clocks, but the advantage is that it's human-listenable, and not going through any computer networks or algorithms between Colorado and you. https://en.wikipedia.org/wiki/Radio_clock#List_of_radio_time_signal_stations https://en.wikipedia.org/wiki/Radio_clock#List_of_radio_time...
- dfc 15y ago"The advantage is that it's human-listenable, and not going through any computer networks or algorithms between Colorado and you." That is an advantage? The Fort Collins signal is awful in the northeast. If you want to deal with the disadvantages of not using advanced computer algorithms to govern your clock i would recommend the USNO's phone service: Time Voice Announcer, Washington, DC: 202-762-1401 & 202-762-1069 (DSN 762-1401, 762-1069) Time Voice Announcer, Colorado Springs, CO: 719-567-6742 (DSN 560-6742)
- rootbear 15y agoSame here. My Dad is a life-long ham radio operator and if I hear a WWV broadcast now, it takes me back to my childhood in a flash. On a related note, I always liked the British time announcements that used three tones, with the designated time being "at the third stroke". It's like a music conductor conducting a measure before you start playing or singing. I wonder if it's measurably more accurate than just playing a tone with the proper time is reached.
- phillmv 15y agoMY understanding of the issue confirms that you would basically need a js implementation of ntp in order for any of this to work. That's why I laughed when Time.is tried to tell me that my clock was off by 1.8 seconds. Barring the sentence above, it just has no way of determining that.
- dfc 15y agoAll things considered it is not such a bad estimate of the time. It looks like it makes it queries the local clock three times to get an estimation of the clock deltas and network delay . Why do you think 1.8 seconds is wrong? What is your ntpq billboard?
- phillmv 15y ago*inspects the page Yeah, you're right. That's not so bad. It just seemed like it was claiming a lot of precision that, from my own experience of javascript, browsers and network latency, seemed a bit over the top. (Granted, on the order of magnitude of seconds, I'll accept that. Less than 100 ms and I grow suspicious).
- dfc 15y agowhat does your machine say for "ntpq -p" and "ntptime"?
- ComputerGuru 15y agoThe answer to your question is GPS devices. They're supposed to take into account (most of) what you mentioned, and even though consumer GPS devices are fairly cheap and not up to industrial or even military GPS, time is one thing they do right. In fact, for several projects that needed a reliable time source instead of using the (as you noticed) poorly implemented atomic clock radio frequencies, we would just buy GPS chips to just extract the time signal. <removed NTP stuff> I simply don't have any clocks that aren't part of electronic devices, my wristwatch being the exception (but it's analog (Roman numerals, even :P) and only has second accuracy anyway, so I don't fuss about it). Simply put, all my devices take care of all this for me. My MacBook and Windows 7 work desktop are both configured to sync to the Apple NTP servers (lowest ping for me, AND they're always up. Microsoft's are down a good 10% of the time in my experience). My cell phone gets its signal from God knows where (iPhone 4S - it could technically be using one of 4 different sources) but is always more or less in sync with the rest (I just checked with time.is for what little that is worth, however, and while my MacBook says that my time is within the margin of error "The difference from Time.is was -0.007 seconds (±0.018 seconds).", my iPhone returns "Your clock is 0.5 seconds slow" - but I would trust my iPhone's NTP implementation over time.is' response as its the same OS X NTP that got it "right" on my MacBook) as it can theoretically use a) GPS b) GSM/CDMA towers c) NTP d) time from PC sync. All my other electronic devices fall in the same category. Garmin nüvi (GPS), Kindle (NTP, auto configured), Cisco IP Phone (NTP, manually configured), Brother printer (NTP, manually configured). I purposely do not set my microwave clock because a) I'll never get it right, and b) any time the power cuts, it resets and in 2012 they're still too stupid to add a 5 cent battery to keep the time in case of power outage.
- dfc 15y agoNTP does a lot more than what you theorized about and none of it is trivial. FYI: Ping is a remarkably bad tool to gauge the accuracy of remote time source. What is the delay value from "ntpq -p" I highly doubt that Apple's time servers are your best bet for time synch. Use the pool.ntp.org servers, preferabbly with your country code: e.g. us.pool.ntp.org, ca.pool.ntp.org.
- ComputerGuru 15y ago
- dfc 15y agoRadio controlled clocks are subject to numerous sources of error. However two seconds is pretty bad, according to NIST the RCC "should always be accurate to within one second of UTC, assuming that they synchronize at least every other day and that their quartz oscillator is of reasonable quality." How far do you live from Fort Collins, CO? I have included some WWVB references at the end... My GPS + Pulse Per Second fed ntp server is remarkably better than a radio controlled clock. The Sure GPS evaluation board probably costs less than your radio controlled clock:) dfc@ronin:~$ ntpq -p remote refid st t when poll reach delay offset jitter ============================================================================== oGPS_NMEA(0) .GPS. 0 l 12 16 377 0.000 0.001 0.001 bonehed.lcs.mit .CDMA. 1 u 32 64 377 51.365 0.670 0.364 rooster.stonybr .CDMA. 1 u 14 64 377 49.019 6.141 0.445 navobs1.oar.net .USNO. 1 u 1 64 377 57.310 9.494 0.456 meg.ee.lbl.gov .PPS. 1 u 1 64 377 88.639 -2.204 0.270 dfc@ronin:~$ ntptime ntp_gettime() returns code 0 (OK) time d3076554.79d48450 Sun, Mar 11 2012 13:54:28.475, (.475899416), maximum error 234 us, estimated error 0 us, TAI offset 34 ntp_adjtime() returns code 0 (OK) modes 0x0 (), offset 0.856 us, frequency -32.941 ppm, interval 1 s, maximum error 234 us, estimated error 0 us, status 0x2001 (PLL,NANO), time constant 4, precision 0.001 us, tolerance 500 ppm, Radio clock reference material: NIST Page on Radio clocks: http://www.nist.gov/pml/div688/grp40/radioclocks.cfm http://www.nist.gov/pml/div688/grp40/radioclocks.cfm "How accurate is a radio controlled clock" http://tf.nist.gov/general/pdf/2429.pdf http://tf.nist.gov/general/pdf/2429.pdf "WWVB Radio Controlled Clocks: Recommended Practices for Manufacturers and Consumers": http://tf.nist.gov/timefreq/general/pdf/2422.pdf http://tf.nist.gov/timefreq/general/pdf/2422.pdf.
- lukeschlather 15y ago>"physical atomic clock" Are you trolling or did you really miss the whole phrase "physical atomic clock synced clocks?"
- dfc 15y agoI apologize. I will edit my post. I did not see the synched the originally and/or his post was edited.
- apaprocki 15y agoIn order to achieve the highest possible accuracy, you have to bring in your own hardware. We use appliances with internal rubidium oscillator clocks which also synchronize with the signal from all visible GPS satellites. This is as good as one can get if you really want to ensure proper time without having an external dependence on a third party or network. edit: A paper from 1989 on the subject: http://tycho.usno.navy.mil/ptti/1989/Vol%2021_14.pdf http://tycho.usno.navy.mil/ptti/1989/Vol%2021_14.pdf
- dfc 15y agoBut you are relying on a third party / network. Your rubidium clock is only good for accurately measuring the length of a second. It has no idea when a UTC second begins. You are relying on GPS time in order to know when the UTC begins. You need time and frequency for "proper time."
- antidaily 15y agoTime.gov is slow and ugly and doesn't work on my phone. Fuck time.gov.