This article carries some netnews discussions on the Timezone support in UNIX SVR4 (also UnixWare). ======================================================================== Subject: TZ Keywords: locales, timezones > I have a SVR4 question. In the /usr/lib/locale/TZ subdirectory in >the SVR4 systems I have seen there exist files which give rules for >locale related timezones, the relationship with GMT, the rules for >daylight savings time and so on. My question relates to the purpose >of these databases and whether some process, function or command makes >use of the information, or if the information must be used manually. With SVR4, System V joins other UNIX systems that have picked up the "Arthur Olson" time zone code, which beats the living hell out of the older SIII/SV time zone code, as well as the kernel-based code from V7 and the variants in older BSDs (newer BSDs have the "Arthur Olson" code). If the TZ environment variable's value begins with ":", the rest of it is treated as a pathname; if it begins with "/", it's an absolute pathname, otherwise it's a pathname relative to "/usr/lib/locale/TZ". If "tzset()" is called explicitly by a program, or implicitly by "localtime()" or "mktime()" being called, and TZ begins with a ":", the file to which TZ refers is loaded up. NOTE: if the files in that directory are text files, they *aren't* the files that "tzset()" loads. "tzset()" loads binary files (in a byte-order-independent and word-size independent format, unless the person who ported the Olson code to a particular implementation screwed up, which they probably didn't, at least on 32-bit machines); the "zic" command compiles the text files into binary files loadable by "tzset()". The timezone/daylight savings time rules are used by "localtime()" to convert UNIX time to local time, and by "mktime()" to convert local time to UNIX time. Any program on the system that uses "localtime()" or "mktime()" should automatically pick up the data from those files - assuming they've been compiled by "zic". (I.e., it won't pick up the data from the text files; you have to run the text files through "zic" first.) The data in the text files may or may not be up to date. Any errors should be forwarded 1) to the vendor of the system and 2) to Arthur Olson, "ado@elsie.nci.nih.gov", although he may just say "that's not the latest version of the file" and send you or point you at the latest data. (The whole reason that stuff *is* in files is so that UNIX machines can cope with legislatures' tendency to change the rules every now and then for various reasons, including, allegedly, "company XXX wants to sell more charcoal briquets, so let's make daylight savings time start earlier in the year so people will barbequeue more often" - and do so without having a complicated TZ variable setting that has to be changed every year, or a complicated TZ variable setting that assumes the rules follow some regular pattern. Except at times when the rules are changed by a government, the Olson code is basically "set it and forget it".) Subject: Re: Daylight Savings time in the UK Keywords: daylight time localtime tgcpwd@rwc.urc.tue.nl (Wim van Dorst) asks ``What is the Olson scheme/code?'' The Olson method is referred to in the Posix standard (see Posix 1003.1-1990 section B.8.1.1). It's an excellent package, and is becoming the standard in modern Unix systems. It uses the TZ environment variable as an index into a table on disk that contains rules that are sensitive to the year to which they are applied. The table is compiled from a human-readable text file. The package can handle a wide variety of time zone anomalies, including double-daylight, leap seconds, different rules for different years, and so forth. If your government changes your timekeeping rules, all you need to do is write up the new rules, run the `zic' program on them, and your programs can use the new rules without recompilation or relinking. You can FTP the Olson code from elsie.nci.nih.gov in pub/tz92.tar.Z. To give you a feeling of what the Olson format rules look like, (and how much politics influences timekeeping!), here are the rules for TZ=GB-Eire as published in tz92. # [British Summer Time] is fixed annually by Act of Parliament. # If you can predict what Parliament will do, you should be in # politics making a fortune, not computing. # Zone NAME GMTOFF RULES/SAVE FORMAT [UNTIL] Zone GB-Eire 0:00 GB-Eire %s 1968 Oct 27 1:00s 1:00 - BST 1971 Oct 31 1:00s 0:00 GB-Eire %s # Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S # 1916 to 1925--irregular Rule GB-Eire 1916 only - Mar 21 1:00s 1:00 BST Rule GB-Eire 1916 only - Oct 1 1:00s 0 GMT Rule GB-Eire 1917 only - Apr 8 1:00s 1:00 BST Rule GB-Eire 1917 only - Sep 17 1:00s 0 GMT Rule GB-Eire 1918 only - Mar 24 1:00s 1:00 BST Rule GB-Eire 1918 only - Sep 30 1:00s 0 GMT Rule GB-Eire 1919 only - Mar 30 1:00s 1:00 BST Rule GB-Eire 1919 only - Sep 29 1:00s 0 GMT Rule GB-Eire 1920 only - Mar 28 1:00s 1:00 BST Rule GB-Eire 1920 only - Oct 25 1:00s 0 GMT Rule GB-Eire 1921 only - Apr 3 1:00s 1:00 BST Rule GB-Eire 1921 only - Oct 3 1:00s 0 GMT Rule GB-Eire 1922 only - Mar 26 1:00s 1:00 BST Rule GB-Eire 1922 only - Oct 8 1:00s 0 GMT Rule GB-Eire 1923 only - Apr 22 1:00s 1:00 BST Rule GB-Eire 1923 only - Oct 16 1:00s 0 GMT Rule GB-Eire 1924 only - Apr 30 1:00s 1:00 BST Rule GB-Eire 1924 only - Sep 21 1:00s 0 GMT # 1925 to 1939 start--regular Rule GB-Eire 1925 1939 - Apr Sun>=15 1:00s 1:00 BST Rule GB-Eire 1925 1938 - Oct Sun>=1 1:00s 0 GMT # 1939 end to 1947--irregular, and with double summer time Rule GB-Eire 1939 only - Nov 19 1:00s 0 GMT Rule GB-Eire 1940 only - Feb 25 1:00s 1:00 BST Rule GB-Eire 1941 only - May 4 1:00s 2:00 DST Rule GB-Eire 1941 only - Aug 10 1:00s 1:00 BST Rule GB-Eire 1942 only - Apr 5 1:00s 2:00 DST Rule GB-Eire 1942 only - Aug 9 1:00s 1:00 BST Rule GB-Eire 1943 only - Apr 4 1:00s 2:00 DST Rule GB-Eire 1943 only - Aug 15 1:00s 1:00 BST Rule GB-Eire 1944 only - Apr 2 1:00s 2:00 DST Rule GB-Eire 1944 only - Sep 17 1:00s 1:00 BST # Double daylight starts on a Monday in 1945? Rule GB-Eire 1945 only - Apr 2 1:00s 2:00 DST Rule GB-Eire 1945 only - Jul 15 1:00s 1:00 BST Rule GB-Eire 1945 only - Oct 7 1:00s 0 GMT Rule GB-Eire 1946 only - Apr 14 1:00s 1:00 BST Rule GB-Eire 1946 only - Oct 6 1:00s 0 GMT Rule GB-Eire 1947 only - Mar 16 1:00s 1:00 BST Rule GB-Eire 1947 only - Apr 13 1:00s 2:00 DST Rule GB-Eire 1947 only - Aug 10 1:00s 1:00 BST Rule GB-Eire 1947 only - Nov 2 1:00s 0 GMT # So much for double saving time. 1948 and 1949, irregular. Rule GB-Eire 1948 only - Mar 14 1:00s 1:00 BST Rule GB-Eire 1948 only - Oct 31 1:00s 0 GMT Rule GB-Eire 1949 only - Apr 3 1:00s 1:00 BST Rule GB-Eire 1949 only - Oct 30 1:00s 0 GMT # 1950 through start of 1953, regular. Rule GB-Eire 1950 1953 - Apr Sun>=14 1:00s 1:00 BST Rule GB-Eire 1950 1952 - Oct Sun>=21 1:00s 0 GMT # 1954 to 1980, starting rules Rule GB-Eire 1954 only - Apr 11 1:00s 1:00 BST Rule GB-Eire 1955 1959 - Apr Sun>=14 1:00s 1:00 BST Rule GB-Eire 1960 only - Apr 10 1:00s 1:00 BST Rule GB-Eire 1961 1963 - Mar lastSun 1:00s 1:00 BST Rule GB-Eire 1964 1967 - Mar Sun>=19 1:00s 1:00 BST Rule GB-Eire 1968 only - Feb 18 1:00s 1:00 BST Rule GB-Eire 1972 1980 - Mar Sun>=16 1:00s 1:00 BST # 1953 to 1971, ending rules Rule GB-Eire 1953 1960 - Oct Sun>=1 1:00s 0 GMT Rule GB-Eire 1961 1967 - Oct Sun>=23 1:00s 0 GMT Rule GB-Eire 1971 only - Oct 31 1:00s 0 GMT # Current rules Rule GB-Eire 1981 max - Mar lastSun 1:00s 1:00 BST Rule GB-Eire 1972 max - Oct Sun>=23 1:00s 0 GMT