Great that Squeak will be in the position to calculate historical
dates properly, so no objection at all.

Thank you for your careful analysis and a synthesis of an algorithm
which corrects the problem. I assume you description will be in a
class comment...

Hannes

On 6/25/12, David T. Lewis <lewis at mail.msen.com> wrote:
> On Mon, Jun 25, 2012 at 01:54:08PM +0200, Bert Freudenberg wrote:
>> Squeak dates are based on the Julian Day Number:
>>
>> 	http://en.wikipedia.org/wiki/Julian_day
>>
>> This is defined as the number of days since January 1, 4713 BC in the
>> Julian proleptic calendar.
>>
>> Now, to my surprise,
>> 	
>> 	Date julianDayNumber: 0
>>
>> answers
>>
>> 	24 November -4713
>>
>> instead of
>> 	
>> 	1 January -4712
>>
>> Digging into this it is because Squeak uses the Fliegel-Van Flandern
>> algorithm, which always results in a Gregorian date. Even for dates before
>> the Gregorian calendar was introduced. If we were to use the algorithm by
>> Jean Meeus, dates before 15 Nov 1582 would be converted to a Julian date.
>>
>> Meeus is a floating point algorithm which we may not want to use. I added
>> a correction to our method similar to Meeus, but converted to integers. So
>> this is a mixed one, pasted below.
>>
>> Would anyone be opposed to adopt this? The behavior will be unchanged for
>> all dates more recent than 15 Nov 1582.
>
> I can't think of any reason why this should cause problems, aside from the
> need to make a couple of trivial updates to the unit tests. Perhaps Brent
> can comment, but it looks to me like a good thing to do.
>
> Dave
>
>
>>
>> If so, we also would need an adjustment for the reverse part in
>> year:month:day:hour:minute:second:nanoSecond:offset:. This is already
>> covered by a test using the year 1200 BC
>> (DateAndTimeTest>>testFromString). Anybody up for making that work?
>>
>> I stumbled upon this because Etoys 5 has a new Calendar object which
>> exposes the Julian Day Number, and setting it to 0 is a very convenient
>> way of explaining what that means.
>>
>> Thanks!
>>
>> - Bert -
>>
>> dayMonthYearDo: aBlock
>> 	"Evaluate the block with three arguments: day month, year.
>> 	Uses the Fliegel-Van Flandern algorithm, with adjustment for dates before
>> 15 Nov 1582 as given by Jean Meeus.
>> 	See http://mathforum.org/library/drmath/view/51907.html"
>>
>> 	| a l n i j dd mm yyyy |
>> 	a := jdn.
>> 	a < 2299161 ifTrue: [ | alpha |
>> 		alpha := (jdn * 4 - 7468865) // 146097.
>> 		a := jdn - 1 - alpha + (alpha // 4)].
>> 	l := a + 68569.
>> 	n := 4 * l // 146097.
>> 	l := l - (146097 * n + 3 // 4).
>> 	i := 4000 * (l + 1) // 1461001.
>> 	l := l - (1461 * i // 4) + 31.
>> 	j := 80 * l // 2447.
>> 	dd := l - (2447 * j // 80).
>> 	l := j // 11.
>> 	mm := j + 2 - (12 * l).
>> 	yyyy := 100 * (n - 49) + i + l.
>>
>> 	^ aBlock
>> 		value: dd
>> 		value: mm
>> 		value: yyyy
>>
>
>