Evergreen pages on precision timekeeping. Revised in place; the feed reflects updates rather than announcing new posts.https://thinkoutsidethebob.com/https://thinkoutsidethebob.com/timekeeping/datacenters/datacenter-gnss-time-best-practices/https://thinkoutsidethebob.com/timekeeping/datacenters/datacenter-gnss-time-best-practices/Eight things worth doing, from the closing slide of a talk on getting the last nanoseconds to UTC. Most cost nothing but attention, and most of the error they remove is invisible until somebody checks.Wed, 12 Aug 2026 00:00:00 GMTGNSSdatacentertimingantennasbuyinghttps://thinkoutsidethebob.com/timekeeping/gnss/limits-of-gnss-time-accuracy/https://thinkoutsidethebob.com/timekeeping/gnss/limits-of-gnss-time-accuracy/The limits are not about radio or geometry. GNSS time is a prediction, so its accuracy is bounded by how good the corrections are — and corrections are forecasts of errors that have not happened yet.Wed, 12 Aug 2026 00:00:00 GMTUTCGNSStimingmetrologyhttps://thinkoutsidethebob.com/timekeeping/datacenters/what-makes-an-accurate-timing-receiver/https://thinkoutsidethebob.com/timekeeping/datacenters/what-makes-an-accurate-timing-receiver/Six traits, in the order they start to matter. The last one separates a box that wants to be your master clock from a box that knows you already have a better one.Wed, 12 Aug 2026 00:00:00 GMTGNSStimingdatacenterbuyingoscillatorshttps://thinkoutsidethebob.com/timekeeping/gnss/choosing-a-gnss-timescale/https://thinkoutsidethebob.com/timekeeping/gnss/choosing-a-gnss-timescale/You are not choosing a receiver so much as choosing whose timescale you end up on. There are three ways to do it, they differ by a factor of a hundred in cost, and most datacenters should take the cheapest.Wed, 12 Aug 2026 00:00:00 GMTUTCGNSStimingdatacenterbuyinghttps://thinkoutsidethebob.com/timekeeping/datacenters/how-gnss-holdover-works/https://thinkoutsidethebob.com/timekeeping/datacenters/how-gnss-holdover-works/When GNSS goes away your clock coasts on the frequency it was last told. How far it drifts is decided by your room more than your oscillator — which is why a holdover measurement and a holdover specification are different objects, and only one of them is a promise.Wed, 12 Aug 2026 00:00:00 GMTGNSStimingdatacenteroscillatorsbuyinghttps://thinkoutsidethebob.com/timekeeping/utc/can-i-build-my-own-link-to-utc-nist/https://thinkoutsidethebob.com/timekeeping/utc/can-i-build-my-own-link-to-utc-nist/The data you would need is public, free, and better than most people realize. You can build the measurement in a weekend. What you cannot build is the one thing that turns it into a number somebody else has to accept.Wed, 12 Aug 2026 00:00:00 GMTUTCGNSStimingmetrologydatacenterhttps://thinkoutsidethebob.com/timekeeping/measuring-time/is-my-clock-right/https://thinkoutsidethebob.com/timekeeping/measuring-time/is-my-clock-right/Timing failures are usually quiet, and nothing inside a wrong clock knows it is wrong. The only detector is a second clock — and the boundary between what the two share and where they are independent decides what you can learn by comparing them.Wed, 12 Aug 2026 00:00:00 GMTdatacentertimingGNSSmonitoringmetrologyhttps://thinkoutsidethebob.com/timekeeping/measuring-time/comparing-distant-clocks/https://thinkoutsidethebob.com/timekeeping/measuring-time/comparing-distant-clocks/You cannot run a cable, and neither clock can be the source for the other. So you both watch a third clock in the sky — which drags its error into your answer. Every technique here is a different way of removing it.Wed, 12 Aug 2026 00:00:00 GMTUTCGNSStimingmetrologyhttps://thinkoutsidethebob.com/timekeeping/questions-to-ask/buying-a-datacenter-gnss-clock/https://thinkoutsidethebob.com/timekeeping/questions-to-ask/buying-a-datacenter-gnss-clock/Nine questions whose answers tell you whether a vendor understands their own product — no test equipment required.Tue, 11 Aug 2026 00:00:00 GMTdatacentertimingbuyinghttps://thinkoutsidethebob.com/timekeeping/time-distribution/time-distribution-protocols/https://thinkoutsidethebob.com/timekeeping/time-distribution/time-distribution-protocols/NTP, NTS, PTP, White Rabbit and a bare PPS, compared on the two things that decide which you need: how close they can get, and whether they account for the length of the wire.Tue, 11 Aug 2026 00:00:00 GMTdatacentertimingPTPNTPWhite Rabbithttps://thinkoutsidethebob.com/timekeeping/questions-to-ask/benchmarking-datacenter-gnss-clocks/https://thinkoutsidethebob.com/timekeeping/questions-to-ask/benchmarking-datacenter-gnss-clocks/You have the hardware and you want to compare it. Most of the traps are in the comparison, not the clocks.Tue, 11 Aug 2026 00:00:00 GMTbenchmarkingtimingmetrologyhttps://thinkoutsidethebob.com/timekeeping/datacenters/acquiring-time/https://thinkoutsidethebob.com/timekeeping/datacenters/acquiring-time/GNSS, a fiber from a national lab, a commercial delivery service, or NTP from the internet. Why GNSS usually wins below 100 ns, and why you want more than one.Mon, 10 Aug 2026 00:00:00 GMTGNSSdatacentertimingUTCWhite Rabbithttps://thinkoutsidethebob.com/timekeeping/utc/can-i-sync-datacenter-clocks-to-utc/https://thinkoutsidethebob.com/timekeeping/utc/can-i-sync-datacenter-clocks-to-utc/Literally no, but practically yes. UTC is computed weeks after the fact, so nothing on Earth can synchronize to it — and yet a well-built datacenter clock sits a few nanoseconds away. Here is how both of those are true.Mon, 10 Aug 2026 00:00:00 GMTUTCtimingmetrologydatacenterhttps://thinkoutsidethebob.com/timekeeping/measuring-time/what-are-timestamps-and-timescales/https://thinkoutsidethebob.com/timekeeping/measuring-time/what-are-timestamps-and-timescales/A timestamp is a count, and a count means nothing until you say what you counted and from when. That is a timescale: tick length, origin, and count — and anything with all three qualifies, including some absurd ones.Mon, 10 Aug 2026 00:00:00 GMTdatacentertimingUTChttps://thinkoutsidethebob.com/timekeeping/measuring-time/how-big-is-a-nanosecond/https://thinkoutsidethebob.com/timekeeping/measuring-time/how-big-is-a-nanosecond/A nanosecond is about a foot. Once the units are physical rather than numerical, most timing arguments get easier to have.Mon, 10 Aug 2026 00:00:00 GMTdatacentertimingmetrologyhttps://thinkoutsidethebob.com/timekeeping/utc/how-gnss-time-relates-to-utc/https://thinkoutsidethebob.com/timekeeping/utc/how-gnss-time-relates-to-utc/Not one relationship but two: a whole number of seconds you can look up, and a sub-nanosecond prediction that two control loops — one running in minutes, one in weeks — spend their lives maintaining.Mon, 10 Aug 2026 00:00:00 GMTUTCGNSStimingmetrologyhttps://thinkoutsidethebob.com/timekeeping/time-distribution/matching-acquisition-and-distribution/https://thinkoutsidethebob.com/timekeeping/time-distribution/matching-acquisition-and-distribution/Two distinct problems that both have to be solved. Their errors sum, so the accurate one is paying for accuracy the other cannot deliver.Mon, 10 Aug 2026 00:00:00 GMTdatacentertimingPTPNTPUTChttps://thinkoutsidethebob.com/timekeeping/gnss/which-constellation-keeps-best-time/https://thinkoutsidethebob.com/timekeeping/gnss/which-constellation-keeps-best-time/Folklore says Galileo. BIPM measures it daily, per constellation, and publishes the answer — and over fourteen months the folklore does not hold up.Mon, 10 Aug 2026 00:00:00 GMTUTCGNSStimingmetrologyhttps://thinkoutsidethebob.com/timekeeping/measuring-time/how-big-is-a-degree/https://thinkoutsidethebob.com/timekeeping/measuring-time/how-big-is-a-degree/Each decimal place of latitude and longitude buys a factor of ten. Knowing where the useful digits stop saves arguing about the ones that cannot mean anything.Mon, 10 Aug 2026 00:00:00 GMTGNSStimingdatacentermetrologyhttps://thinkoutsidethebob.com/timekeeping/utc/who-defines-utc/https://thinkoutsidethebob.com/timekeeping/utc/who-defines-utc/The realizations you can actually reach in real time — and why the laboratories with the best clocks get the most say in what UTC turns out to have been.Mon, 10 Aug 2026 00:00:00 GMTUTCtimingmetrologyhttps://thinkoutsidethebob.com/timekeeping/utc/what-does-traceable-to-utc-mean/https://thinkoutsidethebob.com/timekeeping/utc/what-does-traceable-to-utc-mean/It means an unbroken chain of comparisons back to UTC, each with its uncertainty stated. Not a logo, not a certificate, and not a GPS antenna — which is why most systems that claim it do not have it.Mon, 10 Aug 2026 00:00:00 GMTUTCtimingmetrologydatacenterhttps://thinkoutsidethebob.com/timekeeping/measuring-time/why-precise-time/https://thinkoutsidethebob.com/timekeeping/measuring-time/why-precise-time/Precise time is seldom the goal. It is the price of admission for something else — which is why the requirement usually arrives from outside engineering.Mon, 10 Aug 2026 00:00:00 GMTdatacentertimingtradinghttps://thinkoutsidethebob.com/timekeeping/datacenters/configuring-gnss-antenna-position/https://thinkoutsidethebob.com/timekeeping/datacenters/configuring-gnss-antenna-position/If you don't know your exact location, you can't know the exact time — about a nanosecond of error for every foot you are off vertically. Here is how to get it right.Mon, 10 Aug 2026 00:00:00 GMTGNSSantennastimingdatacenterhttps://thinkoutsidethebob.com/timekeeping/measuring-time/measuring-elapsed-time/https://thinkoutsidethebob.com/timekeeping/measuring-time/measuring-elapsed-time/A clock so bad the internet refuses to talk to it still measures a microsecond-scale interval to about a nanosecond. Short durations forgive both ways a clock can be wrong, and knowing when you are measuring one saves real money.Mon, 10 Aug 2026 00:00:00 GMTdatacentertimingtradingoscillatorsUTChttps://thinkoutsidethebob.com/timekeeping/datacenters/configuring-gnss-antenna-feedline/https://thinkoutsidethebob.com/timekeeping/datacenters/configuring-gnss-antenna-feedline/An uncalibrated feedline is likely your largest source of error to UTC — tens of nanoseconds from a cable nobody measured. Here is how to find the number and configure it.Mon, 10 Aug 2026 00:00:00 GMTGNSSantennastimingdatacenterhttps://thinkoutsidethebob.com/timekeeping/measuring-time/visualizing-precision-trueness-accuracy/https://thinkoutsidethebob.com/timekeeping/measuring-time/visualizing-precision-trueness-accuracy/The most-drawn diagram in metrology, with the axis everyone mislabels put right. Trueness and precision are the two axes; accuracy is the corner where you have both, not a third thing.Mon, 10 Aug 2026 00:00:00 GMTmetrologytimingfigureshttps://thinkoutsidethebob.com/timekeeping/measuring-time/measurement-resolution-and-accuracy/https://thinkoutsidethebob.com/timekeeping/measuring-time/measurement-resolution-and-accuracy/Coarse resolution does not merely limit precision — it counterfeits it. A grid too coarse to show scatter returns the same confident number every time, and stops being able to warn you.Mon, 10 Aug 2026 00:00:00 GMTmetrologytimingfiguresresolutionhttps://thinkoutsidethebob.com/timekeeping/measuring-time/limits-of-averaging/https://thinkoutsidethebob.com/timekeeping/measuring-time/limits-of-averaging/Averaging buys you improvement that stops. What is left when it stops is bias — and the same story plays out in position, in time, and in frequency.Mon, 10 Aug 2026 00:00:00 GMTmetrologyGNSStimingoscillatorshttps://thinkoutsidethebob.com/timekeeping/measuring-time/do-timestamp-errors-average-out/https://thinkoutsidethebob.com/timekeeping/measuring-time/do-timestamp-errors-average-out/No. The archer gets ten arrows and can average them; a network packet is timestamped once. That difference decides which errors you can tolerate — and it moves all the work to before the event.Mon, 10 Aug 2026 00:00:00 GMTdatacentertimingtradingmetrologyhttps://thinkoutsidethebob.com/timekeeping/measuring-time/prioritize-agreement-or-accuracy/https://thinkoutsidethebob.com/timekeeping/measuring-time/prioritize-agreement-or-accuracy/Usually a false choice. The two are not opposed, most improvements buy both, and the real tradeoff only appears at the very last nanoseconds — where it is a genuine engineering preference rather than a confession of low standards.Mon, 10 Aug 2026 00:00:00 GMTdatacentertimingPTPmetrology