On Mon, 31 Oct 2016 08:04:31 -0500, Giliad Wilf wrote: >. >Why do we have to add CVTLDTO (which could be negative if East of GMT), but >subtract CVTLSO? >Because CVTLSO represents by how much TAI is greater than Earth time, and >ETS probably feeds you TAI values. > Whereas CVTLDTO represents how much UTC is less than civil time, the opposite choice of convention.
>Atomic clock ticks ... > All of which I agree with. >When this is about to happen (once in approximately 500 days), we have to >"add" one second to earth clocks, to "postpone" midnight. > ... > "Add" in the sense that the day when that occurs is 86,401 seconds long rather than the ordinary 86,400. Similarly, when Daylight Saving Time ends (next week) we "add" ond hour to our civil time clocks in the sense that that day will be 25 hours long rather than the ordinary 24, by replicating the hour [01:00,02:00). Yet z/OS chooses to subtract one hour from CVTLDTO. >Below you can find animation of two clocks. The upper is TAI (the atomic >clock). >The lower is earth clock. The difference at this moment is 36 (relative to >1957, >but only 26 relative to 1972, which is an alternative epoch on some systems). > z/OS (and I believe GPS) use the 1972 convention. >https://www.timeanddate.com/time/leap-seconds-background.html I suspect the best explanation is historical. OS/360 chose its convention (which I prefer) for CVTLDTO before the advent of UTC. BIPM/IERS independently chose the opposite convention. z/OS chose not to complement the BIPM/IERS published value but only to offset it when setting CVTLSO. The 10-second offset provides continuity at the transition from GMT to TAI. (Topic drift; related to sign conventions and time): I once debated the misdesign of a 10,000 pound rose granite equatorial sundial which was constructed with the hours numbered clockwise on both north and south sides. My opponent argued that it was impossible for the sun to appear to move counterclockwise in the northern hemisphere and clockwise in the southern. The sundial has since been repaired. http://sundials.org/index.php/component/sundials/onedial/172 -- gil ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
