# Julian Dates

I'm running a program where i'm converting dates to julian days.

When I run the formula in D3 i get the correct answer for today 199/01/06. When I rin it in D4 I get the wrong answer with the exact same formula. Why?

function date2julian(date:tdatetime):integer;
var y,m,d:word;
jd:integer;

begin
decodeDate(date,y,m,d);
jd:= ( 1461 * ( y + 4800 + ( m - 14 ) div 12 ) ) div 4 +
( 367 * ( m - 2 - 12 * ( ( m - 14 ) div 12 ) ) ) div 12 -
( 3 * ( ( y + 4900 + ( m - 14 ) div 12 ) div 100 ) ) div 4 +
d - 32075;
date2julian:=jd;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
edit1.text:=inttostr(date2julian(now));
end;
###### Who is Participating?

Commented:
Thank you...  :-)))

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Commented:
Hi
try changing jd to a real not a integer then use FloatToStr()
Regards Barry

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Commented:
Hmmmm. I tested it with Delphi4. The result is 2451185!!!!!!

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Author Commented:

It's very strange. I tried converting to real still no success. Could it meybe be the fact that I'm using NT and not 95.
0

Author Commented:
By the way when I try this.

var d:tdatetime;

d:=now;

The answer for d=36166 for D4 and D3.
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Commented:
Hmmm i just tested  with delphi 3 and delphi 4 on win98 and the
result was 2451185 on both
did you know  on the Delphi Super Page (http://sunsite.icm.edu.pl/delphi/) there is a JDATE component that gives Julian dates,week numbers, etc and it's free .
maybe worth a try
Regards Barry

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Commented:
Hmmm. Please try to split your calculation into several pieces. I mean do this:
(1) Check if "(m-14) div 12" is the same for Delphi3/4.
(2) Check if "(y+4900+(m-14) div 12)" is the same...
(3) and so on...

There must be a difference somewhere, but don't ask me where...

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Commented:
The date you quote in your example is "199/01/06"
Do you mean "1999/01/06" ?

It is possible that you are testing using a date for which the Julian day number is actually negative, and D4 is using a larger word size than D3, or something like that.

Can you define Julian date for me? I believe it's a count of days from a reference date, but what's the reference date?

Also, since TDateTime is merely a fractional count of days from a reference date, can't you just cast to integer and compensate any difference between the Julian reference date and the Windows reference date?
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Author Commented:

I checked the complete formula line by line.

Line1 converts the year to days. Line2 Month to days. Line3 leaves the days.

When I assosiate a variable to each line the error comes up in the first line "the year".

Wamoz
The reference date for Julian is -4713/11/24 which will return 0. What I've decided to do is use your suggestion and add the difference between Julian and TDateTime refence to TdateTime. However, what's worrying is why I get incorrect answer in D4 - will this error affect any other formulas.

Thanks for the help.
Regards Steve
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Commented:
Steve,

jd:= ( 1461 * ( y + 4800 + ( m - 14 ) div 12 ) ) div 4;

jd is integer, so it can hold a value up to 2147483647.
y,m are word, so they can only hold a value up to 65535.
But if you calculate (y+4800)*1461, you'll get the value 9933339. So what does Delphi do with these value? Does Delphi shorten it to word size? Or does Delphi convert the whole calculation to integer size? I am not sure which way is the way it has to be?
If you want the calculation to be restricted to word size you should write "((blabla*blabla) and \$FFFF)". If you want the calculation to run with integer you should do something like this:  "1461 * (integer(y)+4800)".
I don't know why the result of the calculation differs at your computer from Delphi3 to Delphi4 and why it does not differ at my computer. Perhaps there's a hidden compiler parameter somewhere. However, if you are careful with the types, everything should be fine...

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Author Commented:
Guys I found the problem but with no reasons. What I did was broke the formula into the smallest components then checked each vallue in D4 and D3 the conclusion was that D4 refuses to calculate the M-14 part of the formula, which in this case is 1-14=-13.

I have now amended my function as per below; If I substitute the A1 part of the formula with (m-14) I get the wrong answer but when I calculate this part firs I don't have any problems. Could the reason possible be the number of indented brackets that D4 can handle. (I installed D4 on another PC minimum install and had the same problems).

Regards Steve

function date2julian(date:tdatetime):integer;
var y,m,d:word;
jd,YD,MD,DD,e,a1:integer;

begin
decodeDate(date,y,m,d);
a1:=m-14;
YD:= ((1461*(y+4800+((a1) div 12))) div 4);
MD:= ((367*(m-2-12*((a1) div 12))) div 12);
DD:= ((3*((y+4900+((a1) div 12)) div 100)) div 4);
e:=  (d - 32075);
jd:=YD+MD-DD+e;
date2julian:=jd;
end;

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Commented:
I'm quite sure that Delphi has no problems with brackets.

Hmm. Have you installed UpdatePack #2? Perhaps this is the difference between our Delphi4 tests?

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Author Commented:

All is now well.

Thank a lot.
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Commented:
:-)

That's great!

Hmmm. I'm not sure: Shall I answer the question to get the points?
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Author Commented:
0
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