Difference between two dates in Pascal - pascal

Question

This program is supposed to find the difference between two dates but it has a bug and I can't find it.
It keeps returning a big number - please help
Program tp4;
Type
dt = Record
jour : Integer;
mois : Integer;
annee : Integer;
End;
Var
date : dt ;
y,x,i,s : Integer;
Begin
x := 0;
s := 0;
For y:=1 To 2 Do
//2 dates
Begin
Writeln('Entrez un date : jour mois année ');
Readln( date.jour, date.mois, date.annee);
While ((date.jour<=0) Or (date.jour>31) Or (date.mois>12) Or (date.annee<=0) ) Do
//verfication loop
Begin
Writeln('Entrez une date valide : jour mois année ');
Readln(date.jour ,date.mois ,date.annee);
End;
s := s+date.jour ;
For i:= 1 To date.mois-1 Do
Case i Of
3,5,7,8,10,12,1 : s := s+31;
4,6,9,11 : s := s +30;
2 : If ((date.annee Mod 100)=0) And ((date.annee Mod 400) = 0 ) Then //convert months to days
s := s+29
Else If date.annee Mod 4 = 0 Then
s := s+29
Else s := s+28;
End; //Convert years to days
For i:= 1 To date.annee Do
If (i Mod 100 = 0) And (i Mod 400 = 0) Then s := s+366
Else If (i Mod 4 =0) Then s := s+366
Else s := s+365;
x:=s-x ;
End;
If (x)<=0 Then
Writeln('la difference est :',-x)
Else Writeln('la difference est :',x);
Readln;
End.
**input**
12 03 2019
13 03 2019
**output**
737510

Answer 1

I think the error was somewhere in s and x (x:=s-x; etc.), I did refactoring:
Program tp4;
Type dt = Record
jour : Integer;
mois : Integer;
annee : Integer;
End;
Type
arrayDate = array[1..2] of dt;
Var
y,i,f,s : Integer;
arrDate: arrayDate;
function Leap (Y : Word): Boolean;
Begin
If (Y Mod 4) <> 0 Then Leap := FALSE
Else If (Y Mod 100) <> 0 Then Leap := TRUE
Else Leap := (Y Mod 400) = 0;
End;
function Lenght (date: dt) : Integer;
Begin
Lenght := 0;
Lenght := Lenght + date.jour;
For i := 1 To date.mois Do
Case i Of
3, 5, 7, 8, 10, 12, 1 : Lenght := Lenght + 31;
4, 6, 9, 11 : Lenght := Lenght + 30;
2 : If Leap (date.annee) Then Lenght := Lenght + 29 Else Lenght:= Lenght + 28;
End;
For i := 1 To date.annee Do //Convert years to days
If Leap (i) Then Lenght := Lenght + 366 Else Lenght := Lenght + 365;
End;
Begin
For y := 1 To 2 Do //2 dates
Begin Writeln ('Entrez un date : jour mois année ');
Readln (arrDate[y].jour, arrDate[y].mois, arrDate[y].annee);
While ((arrDate[y].jour <= 0)
Or (arrDate[y].jour > 31)
Or (arrDate[y].mois > 12)
Or (arrDate[y].annee <= 0)) Do //verfication loop
Begin
Writeln ('Entrez une date valide : jour mois année ');
Readln (arrDate[y].jour, arrDate[y].mois, arrDate[y].annee);
End;
End;
f := Lenght(arrDate[1]);
s := Lenght(arrDate[2]);
Writeln ('la difference est :', Abs(s - f)); // absolute |s-f|
Readln;
End.

Answer 2

You are using the For y ... loop to request input and to calculate number of days for each input. Within the loop, the s variable is the days counter.
The error you do is that you zero s bfore the For y ... loop and not at the beginning of the loop. Therefore, the second time you request a date, s still has the value from the first date, on top of which you then start to calculate the days for the second date.
The correction is of course to move s := 0; to the beginning of the For y ... loop,
or change the first assignment of s from
s := s + date.jour;
to
s := date.jour;

Answer 3

Finding the difference between two dates 6,000 years apart in a loop? And (ab)using the name of a standard function "Length"? Ouch!
Try this:
//----------------------------------------------------------------------
// Convert a date to its Julian Day Number
//----------------------------------------------------------------------
procedure cj(dd, mm, yyyy: longint; var jdn, dow: longint);
var
cj_y,
cj_c,
cj_x,
cj_y: double;
begin
if dd = 0 then
begin
jdn:= -1
dow:= -1;
end
else
begin
cj_y:= yyyy + (mm - 2.85) / 12;
cj_c:= 0.75 * trunc(cj_y * 0.01);
cj_x:= frac(cj_y);
cj_y:= trunc(cj_y);
jdn:= trunc(
trunc(
trunc(367 * cj_x) + 367 * (cj_y) -
1.75 * cj_y + dd) - cj_c) +
1721115.0;
dow:= jdn mod 7;
end;
end; {cj}
Formula as given is valid for days after 1582-10-15, a small tweak will allow dates going back to 0000-03-01.
Follow the link in https://groups.google.com/g/comp.lang.pascal.borland/c/itwgcfYpLEU which I posted in August 1998 in comp.lang.pascal.borland for explanations.