From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: Andy Wingo Newsgroups: gmane.lisp.guile.bugs Subject: bug#21902: doc incorrectly describes Julian Date Date: Fri, 24 Jun 2016 19:23:48 +0200 Message-ID: <87eg7mcxgb.fsf__38050.9915914901$1466789226$gmane$org@pobox.com> References: <20151113125813.GM13455@fysh.org> NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain X-Trace: ger.gmane.org 1466789226 18374 80.91.229.3 (24 Jun 2016 17:27:06 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 24 Jun 2016 17:27:06 +0000 (UTC) Cc: 21902@debbugs.gnu.org, guile-devel@gnu.org To: Zefram Original-X-From: bug-guile-bounces+guile-bugs=m.gmane.org@gnu.org Fri Jun 24 19:26:57 2016 Return-path: Envelope-to: guile-bugs@m.gmane.org Original-Received: from lists.gnu.org ([208.118.235.17]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1bGUsY-0003aE-Ph for guile-bugs@m.gmane.org; Fri, 24 Jun 2016 19:26:55 +0200 Original-Received: from localhost ([::1]:45153 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bGUsX-0002k7-PB for guile-bugs@m.gmane.org; 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c=nofws; d=pobox.com; h=from:to:cc :subject:references:date:in-reply-to:message-id:mime-version :content-type; q=dns; s=sasl; b=Nze4OI/gV9e0b+Qu7PJvXkJaTvHPLCbp 00N5KfI9Up0NNzjjAieKE7YPYoYs1vvCC7VNHBVGdeifstWFYkvxumbDzxDm3zHx ii4M1fNdNzG+WR/Z9f3srAzVEVsc4HGOTr9WD1Wq5y/kKkwr1noukrb79onYYpxx 6NL5en4sAZM= Original-Received: from pb-sasl1.nyi.icgroup.com (unknown [127.0.0.1]) by pb-sasl1.pobox.com (Postfix) with ESMTP id F27961FEA5; Fri, 24 Jun 2016 13:23:55 -0400 (EDT) Original-Received: from clucks (unknown [88.160.190.192]) (using TLSv1 with cipher ECDHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by pb-sasl1.pobox.com (Postfix) with ESMTPSA id 3B08D1FEA4; Fri, 24 Jun 2016 13:23:55 -0400 (EDT) In-Reply-To: <20151113125813.GM13455@fysh.org> (zefram@fysh.org's message of "Fri, 13 Nov 2015 12:58:13 +0000") User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) X-Pobox-Relay-ID: 6DBBA548-3A30-11E6-A954-C1836462E9F6-02397024!pb-sasl1.pobox.com X-BeenThere: debbugs-submit@debbugs.gnu.org X-Mailman-Version: 2.1.18 Precedence: list X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 208.118.235.43 X-BeenThere: bug-guile@gnu.org List-Id: "Bug reports for GUILE, GNU's Ubiquitous Extension Language" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: bug-guile-bounces+guile-bugs=m.gmane.org@gnu.org Original-Sender: "bug-guile" Xref: news.gmane.org gmane.lisp.guile.bugs:8242 Archived-At: Greets, Time is not my area of expertise :) Cc'ing guile-devel to see if someone can review the ideas. Would you like to propose a specific patch to the documentation? The file is in doc/ref/srfi-modules.texi. Regards, Andy On Fri 13 Nov 2015 13:58, Zefram writes: > The manual says, in the section "SRFI-19 Introduction", > > # Also, for those not familiar with the terminology, a "Julian Day" is > # a real number which is a count of days and fraction of a day, in UTC, > # starting from -4713-01-01T12:00:00Z, ie. midday Monday 1 Jan 4713 B.C. > > There are two errors in the first statement of the epoch for Julian Date, > in ISO 8601 format. The JD epoch is noon on 1 January 4713 BC *in the > proleptic Julian calendar*. The ISO 8601 format is properly never used on > the Julian calendar: ISO 8601 specifies the use of the Gregorian calendar, > including proleptically where necessary (as it most certainly is here). > On the proleptic Gregorian calendar, the JD epoch is noon on 24 November > 4714 BC, and so the ISO 8601 expression should have some "-11-24". > > The second error is in how the year is expressed in ISO 8601. The initial > "-" does not mean the BC era, it means that the year number is negative. > ISO 8601 specifies that the AD era is always used, with year numbers > going negative where necessary; this arrangement is commonly known as > "astronomical year numbering". So "0000" means 1 BC, "-0001" means 2 > BC, and "-4713" means 4714 BC. So the "-4713" is not correct for the > attempted expression of the Julian calendar date, but happens to be > correct for the Gregorian calendar date. > > Putting it together, a correct ISO 8601 expression for the Julian Date > epoch is "-4713-11-24T12:00:00Z". > > The word-based statement of the JD epoch is correct as far as it goes, > but would benefit considerably by the addition of a clause stating that > it is in the proleptic Julian calendar. (Generally, a clarification > of which calendar is being used is helpful with the statement of any > date prior to the UK's switch of calendar in 1752.) The description of > Modified Julian Date is essentially correct. > > However, there's a third problem: misuse of the term "UTC" for historical > times. The description of Julian Date says it's counted "in UTC", > and the statement of the MJD epoch describes its 1858 time as being > specified in UTC. UTC is defined entirely by its relationship to TAI, > which is defined by the operation of atomic clocks. TAI is therefore > only defined for the period since the operation of the first caesium > atomic clock in the middle of 1955. The UTC<->TAI relationship isn't > actually defined even that far back: UTC begins at the beginning of > 1961 (and that was not in the modern form with leap seconds). It is > therefore incorrect to apply the term "UTC" to any time prior to 1961. > These two references to UTC should instead be to "UT", the wider class > of closely-matching time scales of which UTC is one representative. > Also, in the first sentence of this doc section, the phrase "universal > time (UTC)" should be either "universal time (UT)" or (more likely) > "coordinated universal time (UTC)". > > -zefram