[mannykannot]

Thanks for the correction, this is quite important. I had taken statements that Unix and Posix time excludes or ignores leap seconds to mean that it increases monotonically in a continuous manner in sync with whatever atomic clock is the standard; on the contrary, it recognizes and implements leap-seconds with a discontinuity deviating from the steady tick of that standard.

Examples: https://en.wikipedia.org/wiki/Unix_time#Leap_seconds

I also learned that 00:00:00 UTC on 1 January 1970 is a proleptic time, as UTC was not in effect then, though I am not sure that makes it subject to subsequent planetary motions.