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From: Dr J R Stockton on 23 Mar 2010 17:00 I have found some faults in one section of both an ancient and a fairly recent copy of the comp.unix.shell FAQ . Referring to <http://www.chinaunix.net/jh/24/551026.html>, FAQ lines indented as published, my lines begin '**'. 6 d. Determining leap year A leap year in the Gregorian calendar is defined as a year which is evenly divisible by 4, however, if it's also evenly divisible by 100 then it's not a leap year unless it's also evenly divisible by 400. It gets worse than that, actually, but this is as far as I go :-). ** It does not get worse. 4/100/400 represents the full Gregorian rule; ** anything else is at best a proposal. In the Julain calendar which was used before in Europe, only the years divisible by 4 where leap years. ** Spelling : Julian, were. ** It was not used only in Europe, but (at least) in all the various ** European colonies. The standard "cal" utility performed the switch between Julian and Gregorian calendar in september 1752 (see cal 9 1752) which corresponds to the date used in England. The Gregorian calendar ** in Britain and colonies (created by Pope Gregory III) was first used in 1582 in many other countries. ** Gregory XIII. Clavius /et al/ created it; the Pope wanted and ** decreed it. ** Not *many* in 1582. Read The Calendar FAQ. ** ISO Week Numbering and Date of Easter, for which my site has ** algorithms, could be added. -- (c) John Stockton, nr London, UK. ?@merlyn.demon.co.uk Turnpike v6.05. Web <URL:http://www.merlyn.demon.co.uk/> - w. FAQish topics, links, acronyms PAS EXE etc : <URL:http://www.merlyn.demon.co.uk/programs/> - see 00index.htm Dates - miscdate.htm estrdate.htm js-dates.htm pas-time.htm critdate.htm etc.
From: bsh on 23 Mar 2010 22:49 On 2010-03-23, Dr J R Stockton <reply1...(a)merlyn.demon.co.uk> wrote: > I have found some faults in one section of both an ancient and a fairly > recent copy of the comp.unix.shell FAQ. > Referring to <http://www.chinaunix.net/jh/24/551026.html>, FAQ lines > indented as published, my lines begin '**'. This mirror of the FAQ maintained by Joe Halpin, based on the FAQ maintained by, IIRC, Dana French, is now obsolete. The new FAQ, which is already obsolete in regard to missing some discussion of issues regarding features in the newest versions of ksh93, is: http://kornshell.com/doc/faq.html > 6 d. Determining leap year > A leap year in the Gregorian calendar is defined as a year which > is evenly divisible by 4, however, if it's also evenly divisible > by 100 then it's not a leap year unless it's also evenly > divisible by 400. It gets worse than that, actually, but this > is as far as I go :-). > ** It does not get worse. 4/100/400 represents the full Gregorian rule; > ** anything else is at best a proposal. Yes, it gets worse than that :) insofar as there are _three_ contentious algorithms for the calculation of leap year, the stated, and usual one, being the above, being the least accurate. Proposals, to be sure, but a code library _must_ make it clear as to which standard is used to make the calculation. > In the Julain calendar which was used before in Europe, only the > years divisible by 4 where leap years. Sort of. The Julian Calendar, which really should have been called the Cleopatran Calendar, insofar as it was _itself_ a calendrical fix suggested by that ruler, codified by her Alexandrian court astronomer Sosigenes, and mandated by Gaius Julius Optimate -- that is, before he was Caesar -- had further difficulties at its inception because of an error of implementation essentially making the first few leap years every three years, not four. This has to be taken into account by software that does proleptic calendrical calculation, including that for Julian days, by which you probably mean the Modified Julian Day Number, or even the Astronomical Julian Day, being modern refinements of the JDN, which itself increments at noon, not midnight. Indeed, if we are talking about day-day demarcations, which is what the JDN is all about, discussion has to made of the 22+ leap-seconds that have been inserted into the calendar since 1970, which could (or have?) appear on the ISO-standard 61th second (!) of 11:59pm, theoretically erroneously shifting the determination of a given day. Calendrical calculation is _incredibly_ nuanced, and best left to external validated and vetted software tools. The layman would never think of such things as why the length of a day (which is _always_ different) is dependant on the relative time that the northern and southern hemispheres spend in that year's winter! (*) No wonder why the JDN was instigated! > ** Spelling : Julian, were. > ** It was not used only in Europe, but (at least) in all the various > ** European colonies. > The standard "cal" utility performed the switch between Julian > and Gregorian calendar in september 1752 (see cal 9 1752) which > corresponds to the date used in England. The Gregorian calendar > ** in Britain and colonies > (created by Pope Gregory III) was first used in 1582 in many > other countries. > ** Gregory XIII. Clavius /et al/ created it; the Pope wanted and > ** decreed it. > ** Not *many* in 1582. You are correct that this is incredibly naive of the cal(1) software tool. 25 countries and provinces have adopted the Gregorian calendar on 18 different dates, ranging from the year 1582 to as late as 1949. > ** ISO Week Numbering and Date of Easter, for which my site has > ** algorithms, could be added. This should not be part of the FAQ, insofar as it is an user's implementation issue. There are plenty of function libraries for such calculations in shellscript, and mentioning pointers to some is sufficient for inclusion in the FAQ. =Brian (*) Because the majority of tall mountains in the world are in the northern hemiphere, inordinate time spend in winter from one year to the next will mean greater deposits of high-altitude snow and ice, the distal weight of which will slow the earth's revolutions to a very measurable degree!
From: Ed Morton on 24 Mar 2010 00:25 On 3/23/2010 9:49 PM, bsh wrote: > On 2010-03-23, Dr J R Stockton<reply1...(a)merlyn.demon.co.uk> wrote: >> I have found some faults in one section of both an ancient and a fairly >> recent copy of the comp.unix.shell FAQ. >> Referring to<http://www.chinaunix.net/jh/24/551026.html>, FAQ lines >> indented as published, my lines begin '**'. > > This mirror of the FAQ maintained by Joe Halpin, based on the FAQ > maintained by, IIRC, Dana French, is now obsolete. The new FAQ, > which is already obsolete in regard to missing some discussion of > issues regarding features in the newest versions of ksh93, is: > > http://kornshell.com/doc/faq.html > Hmmm, never heard of it. I thought the FAQ was at http://cfajohnson.com/shell/cus-faq.html these days. Ed.
From: Dr J R Stockton on 28 Mar 2010 16:33 In comp.unix.shell message <hoc48j$9f9$1(a)news.eternal-september.org>, Tue, 23 Mar 2010 23:25:54, Ed Morton <mortonspam(a)gmail.com> posted: >On 3/23/2010 9:49 PM, bsh wrote: >> On 2010-03-23, Dr J R Stockton<reply1...(a)merlyn.demon.co.uk> wrote: >>> I have found some faults in one section of both an ancient and a fairly >>> recent copy of the comp.unix.shell FAQ. >>> Referring to<http://www.chinaunix.net/jh/24/551026.html>, FAQ lines >>> indented as published, my lines begin '**'. >> >> This mirror of the FAQ maintained by Joe Halpin, based on the FAQ >> maintained by, IIRC, Dana French, is now obsolete. The new FAQ, >> which is already obsolete in regard to missing some discussion of >> issues regarding features in the newest versions of ksh93, is: >> >> http://kornshell.com/doc/faq.html >> > >Hmmm, never heard of it. I thought the FAQ was at http://cfajohnson.com >/shell/cus-faq.html these days. That seems to match what I was commenting on. Additionally, in "f. Determining the day of the week for a given date.", function dayofweek, consider the line D=$(( $A + $B + $C + $d + $y - ($c * 2) )) That's OK later in each century. But in & soon after March early in a century, the $c term becomes larger than the sum of the rest, and D becomes negative. The next line is echo $(( $D % 7 )) and the result in at least some languages is unsatisfactory for D<0. An efficient remedy is to change - ($c * 2) to + ($c * 5) in the D expression (see my Zeller pages). However, that should be verified in a Unix shell. In the first paragraph of "g. Arbitrary date arithmetic", two "GMT" are required. The error will hit resident Americans when they think it is still 1/18/2038. -- (c) John Stockton, nr London, UK. ?@merlyn.demon.co.uk Turnpike v6.05. Web <URL:http://www.merlyn.demon.co.uk/> - w. FAQish topics, links, acronyms PAS EXE etc : <URL:http://www.merlyn.demon.co.uk/programs/> - see 00index.htm Dates - miscdate.htm estrdate.htm js-dates.htm pas-time.htm critdate.htm etc.
From: Dr J R Stockton on 28 Mar 2010 16:17
In comp.unix.shell message <d8e9a935-8f20-4345-8e5c-dd34e125f2a4(a)y11g200 0prf.googlegroups.com>, Tue, 23 Mar 2010 19:49:50, bsh <brian_hiles(a)rocketmail.com> posted: >On 2010-03-23, Dr J R Stockton <reply1...(a)merlyn.demon.co.uk> wrote: >> I have found some faults in one section of both an ancient and a fairly >> recent copy of the comp.unix.shell FAQ. >> Referring to <http://www.chinaunix.net/jh/24/551026.html>, FAQ lines >> indented as published, my lines begin '**'. > >This mirror of the FAQ maintained by Joe Halpin, based on the FAQ >maintained by, IIRC, Dana French, is now obsolete. The new FAQ, >which is already obsolete in regard to missing some discussion of >issues regarding features in the newest versions of ksh93, is: > >http://kornshell.com/doc/faq.html That is, currently, a dismal document : no date, no named author, and apparently only one shell. > >> 6 d. Determining leap year >> A leap year in the Gregorian calendar is defined as a year which >> is evenly divisible by 4, however, if it's also evenly divisible >> by 100 then it's not a leap year unless it's also evenly >> divisible by 400. It gets worse than that, actually, but this >> is as far as I go :-). >> ** It does not get worse. 4/100/400 represents the full Gregorian rule; >> ** anything else is at best a proposal. > >Yes, it gets worse than that :) insofar as there are _three_ >contentious algorithms for the calculation of leap year, the >stated, and usual one, being the above, being the least accurate. > >Proposals, to be sure, but a code library _must_ make it clear >as to which standard is used to make the calculation. There is only two true standards for the secular Gregorian Calendar : one is the Papal Bull of 1582 (a.k.a. 1581) issued by Gregory XIII (Easter is defined in the Six Canons and the Explicatio of Clavius). A modified calendar could only be Gregorian if it were issued by a later Pope Gregory, or some other authority of that name. The other is the British Calendar Act of 1751 for Leap Year, and its Annex for Easter. Our full secular current calendar is also defined in ISO 8601. Proposals are not standards. Those will not be implemented in our time; and, if they were, the result would not be Gregorian. >> In the Julain calendar which was used before in Europe, only the >> years divisible by 4 where leap years. > >Sort of. The Julian Calendar, which really should have been called >the Cleopatran Calendar, insofar as it was _itself_ a calendrical >fix suggested by that ruler, codified by her Alexandrian court >astronomer Sosigenes, and mandated by Gaius Julius Optimate -- that >is, before he was Caesar -- had further difficulties at its inception >because of an error of implementation essentially making the first >few leap years every three years, not four. This has to be taken into >account by software that does proleptic calendrical calculation, >including that for Julian days, by which you probably mean the >Modified Julian Day Number, or even the Astronomical Julian Day, >being modern refinements of the JDN, which itself increments at >noon, not midnight. Partially true. The Julian Calendar, used from about BC 45, has a Leap Year *every* 4 years. The implementation errors did not affect the calendar of the Emperor Caesar; they merely affected the Roman civil calendar. And more than half of Europe was unaffected; essentially only France, most of Spain, Italy, Greece, and the coast in between in 44 BC, with Switzerland to the Balkans being added by 14 AD. >Indeed, if we are talking about day-day demarcations, which is what >the JDN is all about, discussion has to made of the 22+ leap-seconds >that have been inserted into the calendar since 1970, which could >(or have?) appear on the ISO-standard 61th second (!) of 11:59pm, >theoretically erroneously shifting the determination of a given >day. Only if the day is determined as if by a SI clock. The traditional method is to rely on the alternating light and dark. Before worrying about the occasional +- 1 second, one should consider the Equation of Time, which affected the length of the civil day until good clocks were in use. The only errors deserving correction in that part are the spelling. >Calendrical calculation is _incredibly_ nuanced, and best left to >external validated and vetted software tools. The layman would never >think of such things as why the length of a day (which is _always_ >different) is dependant on the relative time that the northern >and southern hemispheres spend in that year's winter! (*) No >wonder why the JDN was instigated! No; the length of a civil day is 86400 +- 1 SI seconds, +- 0.5 or 1 hour. But that has nothing to do with the calendar, which is a means of labelling a sequence of days independently of how their bounds are determined. >> ** Spelling : Julian, were. >> ** It was not used only in Europe, but (at least) in all the various >> ** European colonies. >> The standard "cal" utility performed the switch between Julian >> and Gregorian calendar in september 1752 (see cal 9 1752) which >> corresponds to the date used in England. The Gregorian calendar >> ** in Britain and colonies >> (created by Pope Gregory III) was first used in 1582 in many >> other countries. >> ** Gregory XIII. Clavius /et al/ created it; the Pope wanted and >> ** decreed it. >> ** Not *many* in 1582. > >You are correct that this is incredibly naive of the cal(1) >software tool. No; I'm not commenting on the tool (and so cannot be right or wrong about it), but on what that FAQ actually says. > 25 countries and provinces have adopted the >Gregorian calendar on 18 different dates, ranging from the >year 1582 to as late as 1949. That cannot be correct. There is now of the order of 200 countries, all using the Gregorian Calendar (some use other calendars as well). ALL have adopted the Gregorian Calendar on or since (approximately, allowing for longitude) JD 2299160.5. The number of countries that changed from Julian to Gregorian is of course smaller. >> ** ISO Week Numbering and Date of Easter, for which my site has >> ** algorithms, could be added. > >This should not be part of the FAQ, insofar as it is an >user's implementation issue. There are plenty of function >libraries for such calculations in shellscript, and >mentioning pointers to some is sufficient for inclusion in >the FAQ. Of course - provided that it is certain that the ones pointed to are both correct and reasonably efficient (preferably directly traceable to the original authority, which is rarely the case for Easter) and that either they are full-range (to within the limits of the machine's arithmetic capability) or that their limits are clearly stated within the code. Be aware that, for example, Microsoft got ISO Week Numbering wrong, and have not corrected the distribution for over half a decade after they put "bug-fix" code (an appalling implementation) on their Web site; and Gauss got Easter wrong at first. So it would be unsafe to assume that anything found on the Web or in libraries is necessarily correct. -- (c) John Stockton, nr London, UK. ?@merlyn.demon.co.uk Turnpike v6.05. Web <URL:http://www.merlyn.demon.co.uk/> - w. FAQish topics, links, acronyms PAS EXE etc : <URL:http://www.merlyn.demon.co.uk/programs/> - see 00index.htm Dates - miscdate.htm estrdate.htm js-dates.htm pas-time.htm critdate.htm etc. |