Dear Colleagues, I would like to split the term "in sync with the rest of the world" in two separate topics.
Hardware Clock Synchronization ============================== The objective of this is to keep the mutual differences of the hardware clocks of a set of machines below some limit. This can be done with ETR, STP, NTP and probably several other protocols. Local Time Synchronization ========================== Assuming that the clocks of a given set of machines are in sync, the objective is to produce the same mapping of hardware clock time - which is usually an unsigned binary integer obtained from a binary counter - to local time (year, month, day, hour, ...) for identical time zones. This already poses a challenge to z/OS, because there are two different sources of time zone information. In the "z/OS UNIX world" the time zone is specified in the TZ environment variable. The information herein contains the UTC offset plus DST rules. TZ remains constant, when switching to and from DST. It only tells functions like localtime(), when to apply DST and when not. This algorithm does not require periodic maintenance of local time. In the "z/OS MVS world" the time zone is stored in CVTLDTO, which only contains the UTC offset. This makes life easy for the TIME macro, which simply adds the offset to UTC to obtain local time. However, CVTLDTO does not remain constant throughout the year. Switching to and from DST literally means to modify CVTLDTO accordingly. In order to keep z/OS UNIX and z/OS MVS local time in sync, CVTLDTO must be adjusted at exactly those points in time, when the DST algorithm of TZ kicks in. NTP considers only UTC and has nothing to do with local time, time zones or daylight saving time. Both ETR and STP allow the specification of an ETS (external time source), to automatically adjust the clock and/or the UTC offset. Again this only modifies CVTLDTO and has no effect on TZ based time. kind regards and have a nice weekend Ing. Gerald Scharitzer, BSc Advanced System Specialist Technology & Tools WAVE Solutions Information Technology GmbH International Services London Member of UniCredit Group 18 Mansell Street, London E1 8AA, UK phone: +44 (0) 20 7170 1977 mobile: +44 (0) 7866 493242 fax: +44 (0) 20 7481 0306 mailto:gerald.scharit...@wave-solutions.com http://www.wave-solutions.com Company name: WAVE Solutions Information Technology GmbH, Company location: Vienna Register of company: Handelsgericht Wien, FN: 122529s Branch: WAVE Solutions Information Technology GmbH International Services London, Reference number: CN FC016222 -----Original Message----- From: IBM Mainframe Discussion List [mailto:ibm-m...@bama.ua.edu] On Behalf Of Gibney, Dave Sent: Thursday, March 12, 2009 9:29 PM To: IBM-MAIN@bama.ua.edu Subject: Re: Time Change (Sync) Clocks are a different story. Twice a year I have to take the sh*t about how hard or expensive it is to have our z9 in sync with the rest of the world. The idea of paying for setting the time is ludicrous to any herder of small boxen. Even though I understand the need for the z/clock to be highly secure, I think IBM does a disservice to all of us who work the platform in small shops. And, I avoid POR like the plague. Got bit this year by an application. Used SET TIMEZONE to spring forward. EntireX servers were an hour off until we noticed when a time critical window opened late. Of course, I don't think even Parallel Sysplex would have helped with this one. ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@bama.ua.edu with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
