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Comment by freehunter | Hacker News Reader

I'm still amazed that computers are so terrible at telling reliable time without connecting to a network constantly. I've seen computers with a fully-functional CMOS battery lose 5 minutes a month without a network connection. Mind blowing.

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That is surprising, so surprising that I have to assume the computer in question was running some kind of misbehaving ntpd that was skewing the clock.

A good quality quartz crystal is accurate to about 15 seconds a month. Even the cheap ones can manage 30 seconds a month.

My car loses about 3 minutes per month, and it has no ability to query NTP. Oddly enough my SUV gains about a minute per month.

It turns out that if you really want a device that can keep accurate time over a long period you must pay a significant amount of money. You would need a temperature compensated crystal (TCXO) or ovenized crystal oscillator (OCXO). Both of these would cost tens to hundreds of dollars, which is orders of magnitude more than hardware makers are willing to spend. Not to mention these kinds of accurate oscillators tend to be power hungry.

But that is missing the main point of the network time checks. Even if computers were perfect at telling time you still need to check network time for security reasons, especially if local users can change the device time. There are crypto attacks that rely on changing the time, and checking that the time matches another somewhat reliable source helps make those attacks harder to pull off.

For a mobile phone it might make sense to check more often as they tend to either move timezones more often, or be powered off and then back on again.

Why would it be surprising? Manufacturers optimize for cost. The clock in your computer is the cheapest one they can find.

Knowing why and how this happens should be a basic course in computer science classes.

I would say its place is more in a computer engineering or an "applied computer science" discipline. Similarly, a chemistry student doesn't especially need to learn about how to plan a safe and cost-effective end industrial synthesis process, which would fall squarely in the domain of chemical engineering.

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