bug#21902: doc incorrectly describes Julian Date

[Zefram]

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