4 ms·

Love this article, but one thing it skipped on was more in-depth on Leap Seconds. You see, UTC, is kinda like another human-made-up timezone. Humans made up s

Love this article, but one thing it skipped on was more in-depth on Leap Seconds. You see, UTC, is kinda like another human-made-up timezone. Humans made up some rules, and UTC is a 37 second offset from TAI / International Atomic Time: https://en.wikipedia.org/wiki/International_Atomic_Time https://en.wikipedia.org/wiki/International_Atomic_Time

milesvp 8y ago
Right? And that that 37 second offset will change from year to year? I was waiting for the bombshell, that some minutes actually have 61 seconds in them, and some have 59 seconds (though it occurs to me that I don't know if astronomers ever add or subtract more than a second for any given clock adjustment).
- welterde 8y ago
  The goal is to always keep UTC within one second of UT1. Short of very drastic events I don't think it will ever be neccessary to introduce more than one leapseconds in a fairly long time interval. And even if it was, I would think that they would do seperate leapseconds events some time apart.
  - jandrese 8y ago
    There are two official primary times a Leap Second can occur. December 31 and June 30. There are provisions for more slots, but they're not really expected to ever be used. Like most things associated with time Leap Seconds are a huge headache to implement properly on computers. If you want some fun watch what various NTP servers around the world do when Leap Seconds roll around. You will see clocks that start to drift for 15 minutes before jumping to the right time, some that wait until 8AM local time on the following day to correct, and some that just go crazy. And of course you have Google's clock smearing across most of a day. IIRC there was one major earthquake that caused a sooner than expected Leap Second adjustment. Fun fact: GPS, NTP, and many similar timing formats have flags in the signal that warn clients of upcoming leap second events. Few receivers pay attention to them however.
    - bcaa7f3a8bbc 8y ago
      > If you want some fun watch what various NTP servers around the world do when Leap Seconds roll around. You will see clocks that start to drift for 15 minutes before jumping to the right time, some that wait until 8AM local time on the following day to correct, and some that just go crazy. And of course you have Google's clock smearing across most of a day. As a note, clock smearing is a non-standard hack invented by Google, because most programs are too broken to handle the leap second warning event and inserting/deleting a second properly, per designed. And you shall never add a clock smearing server to the www.pool.ntp.org.
  - btilly 8y ago
    The Earth's rotation is slowing down. In a few hundred years with the current system, we will need more than 1 leap second per 6 months. How many centuries is hard to tell, since it is affected by many things, including global warming.
    - zimmund 8y ago
      source?
      - welterde 8y ago
        Our moon is slowing down Earth's rotation by tidal friction (angular momentum is transferred from the Earth to the Moon). The effect is rather small with the period only slowing down by around 2ms per 100 years currently (depends on the configuration of the oceans and the orbit of the moon). But internal changes in the Earth can also cause changes in the rotation period. For instance the 2011 Tohoku earthquake shortened the day by 1.8 us [1]. [1] https://www.earthobservatory.sg/blog/how-did-2011-tohoku-earthquake-change-earth%E2%80%99s-rotation https://www.earthobservatory.sg/blog/how-did-2011-tohoku-ear...
      - btilly 8y ago
        https://www.seeker.com/ice-age-clues-help-explain-mysterious-slowdown-of-earths-rotation-2132467383.html https://www.seeker.com/ice-age-clues-help-explain-mysterious... is one of many. Note that if the day slows by 5.5 ms from 24 hours, then we need an average of more than one leap second per 6 months to keep up. Per https://en.wikipedia.org/wiki/Earth%27s_rotation https://en.wikipedia.org/wiki/Earth%27s_rotation the Earth's rotation changed by 1.7 ms in the last century, an average of 2.3 ms in last thousand, and it is being affected by a variety of causes right now.
- unilynx 8y ago
  The 61th second is even in the standards, but it will always be right before UTC midnight: YYYY-MM-DDT23:59:60Z https://tools.ietf.org/html/rfc3339#page-15 https://tools.ietf.org/html/rfc3339#page-15
batbomb 8y ago
Really, it's TAI that should be enough for everyone.
- cryptonector 8y ago
  Yes. Just count seconds and have a handy database of timezones and another of leap seconds. Yes, your computations of future times will be wrong if you can't predict leap seconds accurately (indeed, you cannot!). But if we're talking about a calendar then you want to store wall-clock times and every so often recompute a near-in-the-future time at which to resolve (into TAI) all relevant near-in-the-future times in the calendar so you can fire off reminders/alerts/whatever at the right times.
welder 8y ago
https://developers.google.com/time/smear https://developers.google.com/time/smear
- cryptonector 8y ago
  We don't need more standards. We have UTC and TAI. Pick one, use that one.
  - welder 8y ago
    Leap smearing isn't related to TAI, it's just a way to minimize the effect of leap seconds on machines using NTP servers.
    - cryptonector 8y ago
      Ah, I confused this with a proposal (IIRC discussed on the ietf@ietf.org list a while back) for a new time standard that involves guesstimating leap seconds.
      - welterde 8y ago
        I wonder if it would be possible to just straight-up use UT1 instead of UTC (or is UT1 only available post-facto?). This would essentially be near-infinite smear.
    - deleted 8y ago
      [deleted]
  - Rebelgecko 8y ago
    ... and UT1

milesvp 8y ago

Right? And that that 37 second offset will change from year to year? I was waiting for the bombshell, that some minutes actually have 61 seconds in them, and some have 59 seconds (though it occurs to me that I don't know if astronomers ever add or subtract more than a second for any given clock adjustment).

welterde 8y ago
The goal is to always keep UTC within one second of UT1. Short of very drastic events I don't think it will ever be neccessary to introduce more than one leapseconds in a fairly long time interval. And even if it was, I would think that they would do seperate leapseconds events some time apart.
- jandrese 8y ago
  There are two official primary times a Leap Second can occur. December 31 and June 30. There are provisions for more slots, but they're not really expected to ever be used. Like most things associated with time Leap Seconds are a huge headache to implement properly on computers. If you want some fun watch what various NTP servers around the world do when Leap Seconds roll around. You will see clocks that start to drift for 15 minutes before jumping to the right time, some that wait until 8AM local time on the following day to correct, and some that just go crazy. And of course you have Google's clock smearing across most of a day. IIRC there was one major earthquake that caused a sooner than expected Leap Second adjustment. Fun fact: GPS, NTP, and many similar timing formats have flags in the signal that warn clients of upcoming leap second events. Few receivers pay attention to them however.
  - bcaa7f3a8bbc 8y ago
    > If you want some fun watch what various NTP servers around the world do when Leap Seconds roll around. You will see clocks that start to drift for 15 minutes before jumping to the right time, some that wait until 8AM local time on the following day to correct, and some that just go crazy. And of course you have Google's clock smearing across most of a day. As a note, clock smearing is a non-standard hack invented by Google, because most programs are too broken to handle the leap second warning event and inserting/deleting a second properly, per designed. And you shall never add a clock smearing server to the www.pool.ntp.org.
- btilly 8y ago
  The Earth's rotation is slowing down. In a few hundred years with the current system, we will need more than 1 leap second per 6 months. How many centuries is hard to tell, since it is affected by many things, including global warming.
  - zimmund 8y ago
    source?
    - welterde 8y ago
      Our moon is slowing down Earth's rotation by tidal friction (angular momentum is transferred from the Earth to the Moon). The effect is rather small with the period only slowing down by around 2ms per 100 years currently (depends on the configuration of the oceans and the orbit of the moon). But internal changes in the Earth can also cause changes in the rotation period. For instance the 2011 Tohoku earthquake shortened the day by 1.8 us [1]. [1] https://www.earthobservatory.sg/blog/how-did-2011-tohoku-earthquake-change-earth%E2%80%99s-rotation https://www.earthobservatory.sg/blog/how-did-2011-tohoku-ear...
    - btilly 8y ago
      https://www.seeker.com/ice-age-clues-help-explain-mysterious-slowdown-of-earths-rotation-2132467383.html https://www.seeker.com/ice-age-clues-help-explain-mysterious... is one of many. Note that if the day slows by 5.5 ms from 24 hours, then we need an average of more than one leap second per 6 months to keep up. Per https://en.wikipedia.org/wiki/Earth%27s_rotation https://en.wikipedia.org/wiki/Earth%27s_rotation the Earth's rotation changed by 1.7 ms in the last century, an average of 2.3 ms in last thousand, and it is being affected by a variety of causes right now.
unilynx 8y ago
The 61th second is even in the standards, but it will always be right before UTC midnight: YYYY-MM-DDT23:59:60Z https://tools.ietf.org/html/rfc3339#page-15 https://tools.ietf.org/html/rfc3339#page-15

batbomb 8y ago

Really, it's TAI that should be enough for everyone.

cryptonector 8y ago
Yes. Just count seconds and have a handy database of timezones and another of leap seconds. Yes, your computations of future times will be wrong if you can't predict leap seconds accurately (indeed, you cannot!). But if we're talking about a calendar then you want to store wall-clock times and every so often recompute a near-in-the-future time at which to resolve (into TAI) all relevant near-in-the-future times in the calendar so you can fire off reminders/alerts/whatever at the right times.

welder 8y ago

https://developers.google.com/time/smear https://developers.google.com/time/smear

cryptonector 8y ago
We don't need more standards. We have UTC and TAI. Pick one, use that one.
- welder 8y ago
  Leap smearing isn't related to TAI, it's just a way to minimize the effect of leap seconds on machines using NTP servers.
  - cryptonector 8y ago
    Ah, I confused this with a proposal (IIRC discussed on the ietf@ietf.org list a while back) for a new time standard that involves guesstimating leap seconds.
    - welterde 8y ago
      I wonder if it would be possible to just straight-up use UT1 instead of UTC (or is UT1 only available post-facto?). This would essentially be near-infinite smear.
  - deleted 8y ago
    [deleted]
- Rebelgecko 8y ago
  ... and UT1