4 ms·

I don't think I ever heard of anyone (besides scientists) to store timestamps in TAI. Though Ada supports leap second calculations in the standard library: http

I don't think I ever heard of anyone (besides scientists) to store timestamps in TAI. Though Ada supports leap second calculations in the standard library: http://www.adaic.org/resources/add_content/standards/05rat/html/Rat-7-3.html http://www.adaic.org/resources/add_content/standards/05rat/h...

cryptonector 8y ago
Dealing with time well into the future is rough because of the unknown timezone changes and unknown leap seconds. There's not much you can do. Computing future times in TAI from user inputs in wall clock time is fraught, so I guess you can't store TAI in those cases. For a calendar app, since users want to deal in wall clock time, you have to store time with timezone and with leap seconds (so UTC + time zone). And you have to store timezone, not offset to UTC -- timezones can change. At least internally, however, dealing in TAI can be helpful[0]. Also, IIRC there's a proposal to redefine UTC as without leap seconds[1]... That scares me. Users really need wall clock time. The continued existence of TAI was questioned in a 2007 letter from the BIPM to the ITU-R which stated "In the case of a redefinition of UTC without leap seconds, the CCTF would consider discussing the possibility of suppressing TAI, as it would remain parallel to the continuous UTC."[16] [0] ttps://cr.yp.to/time.html [1] https://en.wikipedia.org/wiki/International_Atomic_Time https://en.wikipedia.org/wiki/International_Atomic_Time
- tialaramex 8y ago
  "That scares me. Users really need wall clock time." Why would getting rid of leap seconds from UTC be a problem for that? The reason there are leap seconds is nothing to do with wall clock time, it's because some people feel the time ought to be intimately connected to the Earth's rotation, but the Earth doesn't oblige by rotating steadily. In my view the people demanding this relationship be maintained ought to take responsibility for fixing it from their side. Speed up or slow down the Earth. Can't? Too bad then, but don't expect us to keep fiddling with the clocks.
  - perl4ever 8y ago
    I don't understand why you say leap seconds have nothing to do with wall clock time. Don't wall clocks include leap seconds?
    - tialaramex 8y ago
      Wall clocks just track UTC. Today they have leap seconds because we defined UTC to include leap seconds, tomorrow if UTC or a replacement universal time no longer has leap seconds then wall clocks won't have leap seconds.
      - cryptonector 8y ago
        But then people will notice a few decades later that noon in UTC-minus-leap-seconds is not noon.
        tialaramex 8y ago
        But it _is_ noon. You are probably thinking of sun transit time, "solar noon", the moment in each day when the sun appears to be "highest" in the sky. This varies of course by position on the Earth, it's how people set "noon" when they didn't need to agree with anybody more than a horse ride's distance away what the time was. So, let's say at least 200 years or more ago. Do you know when solar noon is where you live now? No? Because it's irrelevant. Huge numbers of people live in places where the solar noon changes by an entire hour twice a year for no sensible reason. Does this cause a huge problem? No, there's a slightly elevated rate of road accidents and things like that, but nothing major. A few seconds per decade is _nothing_.
        cryptonector 8y ago
        What's noon? Depends on who defines it. This is the problem with time.
  - deleted 8y ago
    [deleted]
- perl4ever 8y ago
  Times in the future being problematic reminds me of the quote attributed to Neils Bohr: "it is very difficult to predict — especially the future."

cryptonector 8y ago

Dealing with time well into the future is rough because of the unknown timezone changes and unknown leap seconds. There's not much you can do. Computing future times in TAI from user inputs in wall clock time is fraught, so I guess you can't store TAI in those cases. For a calendar app, since users want to deal in wall clock time, you have to store time with timezone and with leap seconds (so UTC + time zone). And you have to store timezone, not offset to UTC -- timezones can change. At least internally, however, dealing in TAI can be helpful[0]. Also, IIRC there's a proposal to redefine UTC as without leap seconds[1]... That scares me. Users really need wall clock time. The continued existence of TAI was questioned in a 2007 letter from the BIPM to the ITU-R which stated "In the case of a redefinition of UTC without leap seconds, the CCTF would consider discussing the possibility of suppressing TAI, as it would remain parallel to the continuous UTC."[16] [0] ttps://cr.yp.to/time.html [1] https://en.wikipedia.org/wiki/International_Atomic_Time https://en.wikipedia.org/wiki/International_Atomic_Time

tialaramex 8y ago
"That scares me. Users really need wall clock time." Why would getting rid of leap seconds from UTC be a problem for that? The reason there are leap seconds is nothing to do with wall clock time, it's because some people feel the time ought to be intimately connected to the Earth's rotation, but the Earth doesn't oblige by rotating steadily. In my view the people demanding this relationship be maintained ought to take responsibility for fixing it from their side. Speed up or slow down the Earth. Can't? Too bad then, but don't expect us to keep fiddling with the clocks.
- perl4ever 8y ago
  I don't understand why you say leap seconds have nothing to do with wall clock time. Don't wall clocks include leap seconds?
  - tialaramex 8y ago
    Wall clocks just track UTC. Today they have leap seconds because we defined UTC to include leap seconds, tomorrow if UTC or a replacement universal time no longer has leap seconds then wall clocks won't have leap seconds.
    - cryptonector 8y ago
      But then people will notice a few decades later that noon in UTC-minus-leap-seconds is not noon.
      - tialaramex 8y ago
        But it _is_ noon. You are probably thinking of sun transit time, "solar noon", the moment in each day when the sun appears to be "highest" in the sky. This varies of course by position on the Earth, it's how people set "noon" when they didn't need to agree with anybody more than a horse ride's distance away what the time was. So, let's say at least 200 years or more ago. Do you know when solar noon is where you live now? No? Because it's irrelevant. Huge numbers of people live in places where the solar noon changes by an entire hour twice a year for no sensible reason. Does this cause a huge problem? No, there's a slightly elevated rate of road accidents and things like that, but nothing major. A few seconds per decade is _nothing_.
        cryptonector 8y ago
        What's noon? Depends on who defines it. This is the problem with time.
- deleted 8y ago
  [deleted]
perl4ever 8y ago
Times in the future being problematic reminds me of the quote attributed to Neils Bohr: "it is very difficult to predict — especially the future."