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. Charles -----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 To: IBM-MAIN@LISTSERV.UA.EDU 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? ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN