The combination of a time and guaranteed error bound can be used as a primitive in distributed systems. You can get timestamps from requests coming from different systems and relate them in a meaningful way. At least, in a false negative fashion. E.g., if A occurs at T + 100ms with 1ms uncertainty and B occurs at T + 105ms with 2ms uncertainty you can conclude that A happened before B. This does not allow you to always establish an order (e.g., if there is overlap) but this isn't always necessary. For instance if instead you are B and you want to have something occur after some time A with uncertainty A_uncertainty, then you can sleep until B with uncertainty B_uncertainty such that B - B_uncertainty > A + A_uncertainty.

None of this has to do with Amazon though. I am very uncomfortable with Amazon (or the others) pretending that they invented precision timekeeping and that it is a special feature only they have because it seems like a lot of people believe it and build systems that rely on an API that is proprietary to a cloud vendor rather than just installing ntp.

Traditional systems don't offer the same guarantees, do they?

Yes they do. See thread above: https://news.ycombinator.com/item?id=29104177

Right, but you won't get a _good_ global error bound without doing the same amount of work. Do you have atomic clocks lying around?

Yes I do, an Rb standard. You can buy them on eBay. It’s completely optional and only has any effect when GPS reception is down.

You must know some pretty smart five year olds!

That doesn't say L5? /s

I missed that part :-)

A person with two watches never knows what time it is.

Nah. They just figure out eventually one of them is kind of wonky and ignore it. Or maybe both are, so they end up just looking at the stove clock.

Also you can't tell a story just with the Aesop ending - you have to tell the fable and END with that line.

I can't find the fable.

I found “Segals Law” https://en.m.wikipedia.org/wiki/Segal%27s_law

Ah, no, the misunderstanding was with something else. Apparently "Aesop ending" is a keyword/crossword definition. It means "the moral of the story" :-)

So what I’m hearing is “Get three or more watches.”.

Time is hard.

By asking both super-microscopic stuff and stuff way out in space, you can now find out what time it is!

One is cesium? Whats the other stuff, way out in space?

Some Pulsars have extremely stable frequencies https://www.nytimes.com/1983/02/09/us/pulsar-termed-most-acc...

Galileo sats use these

https://www.esa.int/Applications/Navigation/Galileo/Galileo_...

so, basically magic.

You think that's magic, check out optical lattice clocks. Blows my mind every time I re-read about them and accept "yes, this actually works".

Rubidium. Cesium atomic clocks are more expensive.

GPS

The GPS constellation.