Topics In Timekeeping Is Live
Two talks in, I kept running into the same problem.
A presentation is a line. You get an hour, you pick a path through the material, and everything off that path gets a wave of the hand — that’s a whole other talk — and then it never is. The questions afterwards were always about the things I had waved at.
So I have written those down instead. Topics In Timekeeping is live.
What it is
Thirty pages on getting precise time into a datacenter and knowing whether you actually have it. It is deliberately not a series of blog posts.
Posts are dated. They are a record of what I thought in a particular week, and the honest thing to do with an old one is leave it alone. These pages are the opposite: they have no publication date, they carry an updated date, and I intend to keep them correct. When I learn something that contradicts one, I will change the page rather than write a new post explaining that the old one was wrong.
That is a promise about maintenance, not about being right the first time. Early drafts of several of these pages carried arguments I withdrew once I checked them properly — including one that was refuted by the very table I had cited in its support.
The numbers
| pages | 30, in six sections |
| words | about 42,000 |
| glossary entries | 32 |
| figures | 19 — 10 drawn for the site, 9 from my talks |
| links between pages | 143 |
| outside sources cited | 22 |
Eleven days from the first page to this one.
The six sections
Asking the Right Questions — what to ask a vendor before you buy a clock, and what to ask yourself before you benchmark one. Nine questions whose answers tell you whether the person selling understands what they are selling.
Timekeeping in Datacenters — the practical core. How to get time into a building, what makes a receiver accurate, what to type in for your antenna position and feedline, how holdover actually works, and eight best practices that mostly cost nothing but attention.
GNSS — what limits GNSS time accuracy, which constellation keeps the best time, and which correction stream you actually need. The short version: GNSS time is a prediction, and that single fact sets every limit.
What Is UTC — whether you can sync to UTC at all (literally no, practically yes), who defines it, what “traceable to UTC” really requires, and whether you could build your own link to UTC(NIST). We tried. The answer is more interesting than yes or no.
Measuring Time — the metrology underneath all of it. Timestamps and timescales, precision versus trueness, why averaging stops helping, why resolution can counterfeit precision, and how you would know your clock was still right.
Time Distribution — NTP, NTS, PTP, White Rabbit and a bare PPS, compared on the two things that decide which you need, and why acquiring time well and distributing it badly wastes the money you spent on the first part.
There is also a glossary, because half of this subject is people using the same word for different things.
Where to start
Depends what brought you here.
- Somebody handed you a number and told you to hit it → Why do you want precise time?
- You are buying → Questions to ask a clock vendor
- You already have clocks and want them better → Datacenter GNSS time best practices
- You want the idea that reorganized my own thinking → What are the limits of GNSS time accuracy?
Almost every failure in this field is quiet. A wrong antenna position, an uncompensated cable, a receiver surveying itself badly, a clock drifting after its reference vanished — none of them raise an alarm, none degrade a status light, and all of them produce a confident, steady, wrong answer.
Which is why so many of these pages end up in the same place: the number you can defend is the one you checked against something independent, and wrote down.
It will change
It is a garden, not a stream. Pages will be revised, split, retitled and occasionally deleted. There is an RSS feed if you want to hear about that.
Several pages carry measurements I have not finished making, and I have tried to be explicit about which numbers are mine, which are somebody else’s, and which are modelled rather than measured. If you spot something wrong, I want to know — the About page says how to reach me, and getting it right matters to me more than having said it first.