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    $\begingroup$ What particular problem, if any, would there be with selecting one of two or three lengths of "civil second", which differ by about 0.05ppm, on January 1 of each year? The only things that would need to care about the varying length of a civil second would be programs that need to convert between civil time and atomic time, or those that keep civil time and have their own time base which is accurate to better than 0.05ppm. Having the rate at which time flows change by 0.05ppm would seem much less disruptive than having some "five-second" intervals be a whole second longer. $\endgroup$ Commented Apr 23, 2018 at 22:31
  • $\begingroup$ @supercat thats an interesting question. I recall google used a similar scheme recently to handle leap seconds internally. I suggest posting it as a seperate question though. I'm not sure which would be the correct stackexchange though. $\endgroup$ Commented Apr 24, 2018 at 7:04
  • $\begingroup$ @supercat: The nightmare scenario is that some jurisdictions adopt your idea and some don't, so you need to keep this info in zoneinfo files and crossing timezones changes the time by +-1h (or whatever) and retroactively do a leap second and undo the accumulation or something? I think and hope that's too obviously horrible for anyone to consider. 0.05 ppm is lost in the imprecision of most real clocks, so computers would typically just use NTP exactly as before with some multiplier on the local clock rate. But stuff with precise clocks could maybe have their assumptions violated. $\endgroup$ Commented Apr 24, 2018 at 8:55
  • $\begingroup$ @PeterCordes: I suppose that perhaps a slight improvement to my rule would be to say that 180 days starting on Jan 1 and July 1 would be adjusted (or not) by 1/16,552,000 and the remaing days wouldn't. That would allow both times to precisely coincide on Jan 1. and July 1 of every year. Having some references use leap seconds and some not wouldn't cause problems if systems that expect leap seconds consistently use references that use it. Systems that don't use leap seconds are going to have unavoidable deviations from leap-second time no matter what they do, but... $\endgroup$ Commented Apr 24, 2018 at 14:53
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    $\begingroup$ ...they could have less deviation if they were synchronized to a clock that smeared leap seconds over 180 days (I think Google's leap smear spreads the leap second over one day, but smearing it over 180 would seem even better). $\endgroup$ Commented Apr 24, 2018 at 14:54