[LEAPSECS] Leap second mess at NASA

[Tony Finch]

Zefram <zefram at fysh.org> wrote:

>

> "AD 0" is a perfectly meaningful label.  It refers to the year before AD

> 1; that year is also known as 1 BC.  Likewise, AD -1 is 2 BC, and so on.

> This kind of year numbering is common in astronomy, and was written into

> the latest version of ISO 8601.  The use of AD year numbers below 1 is,

> of course, contrary to convention in less technical contexts.


It's called the "proleptic Gregorian calendar". It is described in both
ISO 8601:2000 and ISO 8601:2004. Oddly they removed the clarification
about the difference from the AD/BC system in the 2004 edition.

ISO 8601:2000

4.3.2.1 The Gregorian calendar

This International Standard uses the Gregorian calendar for the
identification of calendar days.

The Gregorian calendar provides a reference system consisting of a,
potentially infinite, series of contiguous calendar years. Consecutive
calendar years are identified by sequentially assigned year numbers. A
reference point is used which assigns the year number 1875 to the calendar
year in which the "Convention du Mètre" was signed at Paris.

The Gregorian calendar distinguishes common years with a duration of 365
calendar days and leap years with a duration of 366 calendar days. A leap
year is a year whose year number is divisible by four an integral number
of times. However, centennial years are not leap years unless they are
divisible by four hundred an integral number of times.

This International Standard allows the identification of calendar years by
their year number for years both before and after the introduction of the
Gregorian calendar. For the determination of calendar years and year
numbers only the rules mentioned above are used. For the purposes of this
International Standard these rules are referred to as the Gregorian
calendar. The use of this calendar for dates preceding the introduction of
the Gregorian calendar (i.e. before 1582) should only be done by agreement
of the partners in information interchange.

NOTE 1 In the prolaptic Gregorian calendar the calendar year [0000] is a
leap year.

NOTE 2 No dates shall be inserted or deleted when determining dates in the
prolaptic Gregorian calendar (this may be necessary for the calculation of
dates in the Julian calendar before 1582). Also note that the year numbers
of years before the calendar year [0001] differ from the year numbers in
the "BC/AD calendar system", where the year "1 BC" is followed by the year
"1 AD".

In the Gregorian calendar each year is divided in 12 sequential calendar
months as indicated in Table 1.

[table 1 follows]

ISO 8601:2004

3.2.1    The Gregorian calendar

This International Standard uses the Gregorian calendar for the
identification of calendar days. This calendar provides a time scale
consisting of a, potentially infinite, series of contiguous calendar
years. Consecutive calendar years are identified by sequentially assigned
year numbers.

The Gregorian calendar distinguishes common years of 365 consecutive
calendar days and leap years of 366 consecutive calendar days. A leap year
is a year whose year number is divisible by four an integral number of
times. However, a centennial year is not a leap year unless its year
number is divisible by four hundred an integral number of times.

In the Gregorian calendar each calendar year is divided in 12 sequential
calendar months, each consisting of a specific number of calendar days as
indicated in Table 1.

The Gregorian calendar has a reference point that assigns 20 May 1875 to
the calendar day that the "Convention du Mètre" was signed in Paris.

This International Standard allows the identification of calendar years by
their year number for years both before and after the introduction of the
Gregorian calendar. For the determination of calendar years, the year
number and the calendar day within the calendar year only the rules
mentioned above are used. For the purposes of this International Standard
the calendar based on these rules is referred to as the Gregorian
calendar. The use of this calendar for dates preceding the introduction of
the Gregorian calendar (also called the proleptic Gregorian calendar)
should only be by agreement of the partners in information interchange.

The introduction of the Gregorian calendar included the cancellation of
the accumulated inaccuracies of the Julian calendar. However, no dates
shall be inserted or deleted when determining dates in the proleptic
Gregorian calendar.

NOTE In the proleptic Gregorian calendar, the calendar year [0000] is a
leap year.

EXAMPLE The Gregorian calendar was introduced on 15 October 1582. In the
calendar set by this standard the calendar day preceding that calendar day
is referred to as 14 October 1582. In the Julian calendar that calendar
day is referred to as 4 October 1582.

[table 1 follows]

Tony.
-- 
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