From: Andy Wingo <wingo@pobox.com>
To: Zefram <zefram@fysh.org>
Cc: 21902@debbugs.gnu.org, guile-devel@gnu.org
Subject: bug#21902: doc incorrectly describes Julian Date
Date: Fri, 24 Jun 2016 19:23:48 +0200 [thread overview]
Message-ID: <87eg7mcxgb.fsf__38050.9915914901$1466789226$gmane$org@pobox.com> (raw)
In-Reply-To: <20151113125813.GM13455@fysh.org> (zefram@fysh.org's message of "Fri, 13 Nov 2015 12:58:13 +0000")
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 <zefram@fysh.org> 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
next prev parent reply other threads:[~2016-06-24 17:23 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-13 12:58 bug#21902: doc incorrectly describes Julian Date Zefram
2016-06-24 17:23 ` Andy Wingo [this message]
[not found] ` <87eg7mcxgb.fsf@pobox.com>
2016-06-24 18:01 ` Zefram
[not found] ` <20160624180143.GI1170@fysh.org>
2016-06-25 9:36 ` bug#21902: reviewer for time-related bugs (srfi-19) Andy Wingo
2018-10-20 21:46 ` bug#21902: doc incorrectly describes Julian Date Mark H Weaver
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
List information: https://www.gnu.org/software/guile/
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='87eg7mcxgb.fsf__38050.9915914901$1466789226$gmane$org@pobox.com' \
--to=wingo@pobox.com \
--cc=21902@debbugs.gnu.org \
--cc=guile-devel@gnu.org \
--cc=zefram@fysh.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).