I'm finding it difficult to approach your certainty on this. By convention, considering only UTC, leap seconds are inserted between days. This means that, for UTC, there is no conflict between your EXACTLY assertions and the occasional leap second insertion. But according to [Wikipedia on this](https://en.wikipedia.org/wiki/Leap_second#Insertion_of_leap_seconds), leap seconds are inserted "simultaneously" (in the everyday sense) across the world. This would mean that for timezones other than GMT, some days will be a little longer than 60^2 * 24 seconds. At [Wikipedia's article on Julian days](https://en.wikipedia.org/wiki/Julian_day), this issue is touched upon thusly: "Time intervals calculated from differences of Julian Dates specified in non-uniform time scales, such as UTC, may need to be corrected for changes in time scales (e.g. leap seconds)." I doubt that the problem is avoided by using timezones other than UTC. I suppose we could agree that Julian day calculations are entirely independent of leap seconds, by fiat, then pronounce that differences between Julian days are to be given astronomical accuracy by a post-adjustment process. Still, I think an asterisk may be needed in your set of EXACTLY assertions as applied to the present thread's nominal topic. As for "leap seconds are artifices": Yes, as is quantified time in general.