Comment by harmil | Hacker News Reader

Comment by harmil | Hacker News Reader

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ntpd guarantees (under the default settings now, though that's new... you can ask it to shoot you in the head, but I don't recommend that you use firearms in this way) that the clock will never go backwards. You're thinking of nptdate, a utility provided with ntpd, but which is only ever supposed to run at system initialization time to get the time close enough for ntpd to take over.

If ntpd finds that the clock is ahead, it will slow the ticks, allowing it to gradually drift back to the correct time.

The system time should NEVER move backwards. Ever. Relying on the system time not moving backwards is not unreasonable for trivialities like how long you wait for a keystroke. If you reset your system time, you might have to hit a key to get vim to wake up. Shucks.

ntpd will step backward if error stays above a configurable threshold for a configurable period of time. We saw it happen after the recent leap second. 1 second exceeds the default step threshold of 128 ms. With Linux limiting the slew rate to 500 ppm, gradual correction would have taken 2000 seconds, and by default nptd steps after 900 seconds.

> ntpd guarantees (under the default settings now [...]) that the clock will never go backwards.

Can you elaborate on what changed? I just downloaded the latest reference implementation, and the man page still seems to indicate that the default behavior is to step when error > 128ms for a prolonged period.

There is no such guarantee when using CLOCK_REALTIME. If you need this guarantee, use the monotonic clock. I've had issues with graphics engines written naively assuming nobody would change CLOCK_REALTIME. It's a bad assumption because many embedded systems well not use ntp and will instead rely on a user to set the clock. It may go backwards if the user sets it backwards. It may even do so while your software is doing something like presenting the user with a UI to set the time.

With Linux/NTP there was two times 23:59:59 UTC not long ago: https://access.redhat.com/articles/15145

The clocks go back an hour every year here in (most of) Australia. Every year, on a specific date 1am-2am happens twice.

Do you mean daylight saving time? The time zone changes, but Unix time is UTC so it's unaffected.

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