bug#21902: doc incorrectly describes Julian Date

[Mark H Weaver <mhw@netris.org>]

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)".

I changed the text, based partly on your proposed patch and partly based
on similar recent fixes in the upstream SRFI-19 document, in commit
5106377a3460e1e35daf14ea6edbe80426347155 on the stable-2.2 branch.

I'm closing this bug now, but feel free to reopen if there are still
problems.

     Thanks!
       Mark