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Computers usually report time in UTC but don't actually have leap seconds. Most computers are pretty bad at keeping time, and regularly sync to NTP servers for
by dexwiz 5y ago
Computers usually report time in UTC but don't actually have leap seconds. Most computers are pretty bad at keeping time, and regularly sync to NTP servers for the current time. NTP servers generally have better time keeping hardware (or sync with better sources). When a leap second occurs, NTP servers will smear that second over 12 hours or so. Different NPT servers have different smear standards. During that smear, seconds are a bit longer, or shorter for negative leap seconds, than usual. But for general computing this doesn't matter. The smear is on a similar order of magnitude of normal clock drift. This isolates the leap second to only computers that need to track it. UTC/UT1 is designed to be used by humans and is based on the Earth's revolution and rotation about the sun (well now it's actually measured using distant pulsars).
The big reason to isolate this is because leap seconds aren't totally determinate. We can predict them to some degree, but ultimately rely on measurement. Unlike something like leap days, you cannot safely code a system that accounts for future leap seconds. Leap days follow a well-defined formula, leap seconds do not.
For time sensitive applications like navigational computers, TAI is used. TAI is currently 37 seconds off from UTC. TAI is associated with the SI Second unit. While originally derived from the solar day, it is now based on the vibrational frequency of a caesium atom.
For a more detailed explanation see http://mperdikeas.github.io/utc-vs-ut1-time.html http://mperdikeas.github.io/utc-vs-ut1-time.html for a good summary.
For an overview on how Google handles the smear see https://developers.google.com/time/smear https://developers.google.com/time/smear
- staticautomatic 5y agoHow can air-gapped computers keep time well?
- zozbot234 5y agoUse a GPS receiver, that will only get time reference and location from the GPS satellites, and not transmit anything. So you're still airgapped - see https://gpsd.gitlab.io/gpsd/gpsd-time-service-howto.html https://gpsd.gitlab.io/gpsd/gpsd-time-service-howto.html for reference.
- onionisafruit 5y agoIs it really still airgapped if it accepts input from an outside system?
- fprotthetarball 5y agoNo data leaves the computer over GPS and the computer controls how it processes the data. It's no worse than a keyboard.
- matthewmacleod 5y agoIt's not really an air-gapped system if it accepts unauthenticated external radio signals and changes state in response, which is what would be happening in that case. You don't need much imagination to think of various attacks that a spoofed GPS signal could open up, if you were able to cause a system to have an incorrect time.
- vba616 5y ago>When a leap second occurs, NTP servers will smear that second over 12 hours or so Since when? I mean that literally, I only heard of this concept recently and didn't think it was a general standard, yet.
- sla29970 5y ago"My system clock is wrong, what action shall I take to correct it?" Common answers to that question are "step" or "smear". The notion that the clock could be wrong is not addressed in the design of most systems, and "smear" is widely accepted as the least harmful fix.
- vba616 5y agoYes, I'm wondering when/how "smearing" became widely accepted, since the Wikipedia page on leap seconds only says that there is a proprietary method that Google uses, and another that Amazon uses. "UTC-SLS was proposed as a version of UTC with linear leap smearing, but it never became standard"
- sla29970 5y agoLack of agreement; "smearing" is completely normal for answering "What time is it?", and completely wrong for applications requiring precise time.
- omegalulw 5y agoWhy can't we just switch to TAI for computers and anything electronic everywhere and just use UTC for user visible timestamps?