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Comment by ReactiveJelly | Hacker News Reader

Can't say too much but I saw an IoT product where, if NTP failed, they would all slowly fall behind. I really appreciated this because fixing NTP would jump forward, leaving a gap in perceived time instead of living the same moment twice.

So I assumed that, like how speedometers purposely read a little high, the crystals must purposely read a little slow so that computers don't slip into the future.

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I worked on an embedded video system once where no one took into account the slight difference between the encoder and decoder clock frequencies. If the encoder was slower, no problem, you dropped a frame every once in a while. If the encoder was faster, you would buffer frames up to the memory limit of the system. Since these systems were typically left on for weeks at a time, the pilots would eventually discover the video they were using to fly a vehicle was several seconds in the past. Luckily this was discovered before initial shipment and the code was changed to drop everything in the buffer except the latest video frame.

That's a neat way of ensuring that time never jumps backwards on your system!

Reminds me of the same idea but applied in the opposite way in some train station clocks: Their second hands take slightly less than a minute to complete one rotation, after which they stop and wait for a signal sent from a central clock to be released simultaneously.

Making a clock run slightly slow or fast is much easier than making it run just about correctly :)

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