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.

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.

What does your favorite macro do about this? I have no idea. 


-----Original Message-----
From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On Behalf 
Of Paul Gilmartin
Sent: Friday, February 23, 2018 6:59 AM
Subject: Re: Two summer time change questions

On Fri, 23 Feb 2018 06:42:17 -0800, Charles Mills wrote:

>Thanks all. Yes, I'm well aware of the points that Skip and Allan make. 
>Question one was mostly curiosity. What I really needed to understand 
>was the "timeliness" of CVTLDTO and I have that now, thanks to an 
>off-list reply. (Yes, it is updated immediately when the local offset 
>is changed, either by command or by the ETR.)
Is there a "local offset" other than CVTLDTO, or are they the same thing?

I suspect a minuscule timing window if CVTLDTO is updated between a program's 
(even OS code's) STCK and fetching CVTLDTO.  This could result in an error of 
an hour or an absurd local time result.  Does the code underlying the TIME 
macro account for this?

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