On Fri, 23 Feb 2018 07:24:36 -0800, Charles Mills wrote:

>There are two more or less redundant fields in the CVT: CVTTZ and CVTLDTO. 
>(CVTLDTO is actually in CVT extension 2.) CVTLDTO contains the exact local 
>time offset in timer units -- in other words, you can add it directly to an 
>STCK value. CVTTZ is identical to the high order word (only) of CVTLDTO -- in 
>other words, it is the *approximate* offset in units of 1.048576 seconds. 
>(Approximate because you cannot represent an integral number of hours exactly 
>in units of 1.048576 seconds.) Why 1.048576? One way of looking at that value 
>is that it is 2^20 divided by 10^6. It is the ratio between mibi and mega.
An IBM Redbook has called this a megamicrosecond.  Ugh!  Humpty Dumpty!

>Are there possible timing windows? Yes, all over the place. (Between a STCK 
>and inspecting CVTLDTO? Yes, a small window. What about someone else's STCK 
>that is somehow passed to you and your inspecting CVTLDTO? Potentially a 
>window of seconds, days or even years.) I have come to the conclusion that 
>local time is something that it is impossible to deal with perfectly. You deal 
>with it as well as you can.
It can be done a lot better than x/OS does it, and with no timing hazard:

Dismayingly, provincially, z/OS shuns the Olson database.  NIH!?

-- gil

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