[SAI_Peregrinus]

Much more practical but much less accurate than an atomic wristwatch[1]. I wonder how small a cesium frequency standard could be made. Miniaturization isn't typically a priority in their design, there's surely room for improvement.

[1] http://www.leapsecond.com/pages/atomic-bill/ (Yes, that is a guy with an HP-5071A strapped to his arm.)

[makomk]

High-precision quartz wristwatches with well-designed temperature compensation can get accuracy within single-digit seconds of drift per year, and even that level of accuracy is enough to make them a niche product. Also, you need pretty good battery life in order to ensure the battery doesn't run out before the drift becomes relevant - some of the higher-end quartz movements already struggle with this a little, and chip-scale atomic clocks are much worse and likely to remain that way. Having accuracy of a second in 5,000 years doesn't give you much benefit over quartz because the battery life is measured in days.

[SamReidHughes]

It's actually been shrunk down a bit:

http://www.ablogtowatch.com/bathys-cesium-133-atomic-wrist-w...

https://www.hoptroff.com/collections/atomic-timepieces

I think they all use the same "chip".

[y04nn]

You can also get a radio controlled watch

[SAI_Peregrinus]

Of course, or a GPS-synchronized smart watch. But where's the fun in that?