31 28 31 30 31 30 31 31 30 31 30 31
31 28 31 30 31 30 31 31 30 31 30 31
31 28 31 30 31 30 31 31 30 31 30 31
31 29 31 30 31 30 31 31 30 31 30 31

and, remember, 4 years can be count as 1461 days.

----------------------------------------------------------------------------
---------------------------------------------------------------
Unit Dates;
{Author: Jose Santos <jcsan...@iname.com>}

Interface

Type
 TSmallInteger = Integer;
 TLongInteger  = Longint;

 TDate=Record
        Year  : TSmallInteger;
        Month : TSmallInteger;
        Day   : TSmallInteger;
      End;

Const
 RefDate: TDate=
  (
   Year  :  1000;
   Month :     1;
   Day   :     5
  ); {Reference date.It was a Sunday}

Function Bissext(Year: TSmallInteger): Boolean;
Function DaysInMonth(Year, Month: TSmallInteger): TLongInteger;
Function DaysFromDateXToY(DateX, DateY: TDate): TLongInteger;
Function DayOfWeek(Date: TDate): TSmallInteger;

Implementation

Function Bissext(Year: TSmallInteger): Boolean;
 Begin
  Bissext:=(Year Mod 4=0)And(Year Mod 100>0)Or(Year Mod 400=0);
 End;

Function DaysInMonth(Year, Month: TSmallInteger): TLongInteger;
 Begin
  If Month In [1,3,5,7,8,10,12] Then
   DaysInMonth:=31 Else
  If Month <> 2 Then
   DaysInMonth:=30 Else
  If Bissext(Year) Then
   DaysInMonth:=29 Else
   DaysInMonth:=28;
 End;

Function DaysFromYearXToY(YearX, YearY: TSmallInteger): TLongInteger;
Var
 ResTemp: TLongInteger;
 i: TSmallInteger;
 Begin
  ResTemp:=0;
  For i:=YearX To YearY Do
   If Bissext(i) Then
    ResTemp:=ResTemp+366 Else
    ResTemp:=ResTemp+365;
  DaysFromYearXToY:=ResTemp;
 End;

Function DaysFromMonthXToY(Year, MonthX, MonthY: TSmallInteger):
TLongInteger;
Var
 ResTemp: TLongInteger;
 i: TSmallInteger;
 Begin
  ResTemp:=0;
  For i:=MonthX To MonthY Do
   ResTemp:=ResTemp+DaysInMonth(Year, i);
  DaysFromMonthXToY:=ResTemp;
 End;

Function DaysFromDateXToY(DateX, DateY: TDate): TLongInteger;
Var
 ResTemp: TLongInteger;
 Begin
  If DateX.Year<>DateY.Year Then
   ResTemp:=DaysInMonth(DateX.Year, DateX.Month)-DateX.Day+
            DaysFromMonthXToY(DateX.Year, DateX.Month+1, 12)+
            DaysFromYearXToY(DateX.Year+1, DateY.Year-1)+
            DaysFromMonthXToY(DateY.Year, 1, DateY.Month-1)+
            DateY.Day Else
  If DateX.Month<>DateY.Month Then
   ResTemp:=DaysInMonth(DateX.Year, DateX.Month)-DateX.Day+
            DaysFromMonthXToY(DateX.Year, DateX.Month+1, DateY.Month-1)+
            DateY.Day Else
   ResTemp:=DateY.Day-DateX.Day;
  DaysFromDateXToY:=ResTemp;
 End;

Function DayOfWeek(Date: TDate): TSmallInteger;
{Returns the day of week.0 is for Sunday, 1 for Monday, etc}
 Begin
  DayOfWeek:=DaysFromDateXToY(RefDate, Date) Mod 7;
 End;

Begin
 {No initialization required}
End.

----------------------------------------------------------------------------
-----------------------------------------------------------

You can also easily do a calendar with this unit.But avoid using DayOfWeek
to calculate every day of the month.It takes time and is unnecessary.You
just need to know the first day of the month.
RefDate is only used to calculate weekdays.It's unnecessary if you won't use
DayOfWeek.

Hope this is useful.

Regards,

A program calculating the position of Sun and Moon
(DOS, with source (TPascal) and these two procedures):
http://nest.sikornik.gliwice.pl/~piotrcf/eclip/sky.zip

Other programs:

Sun and Moon: when do they rise and set (DOS):
http://nest.sikornik.gliwice.pl/~piotrcf/eclip/sunr4.zip

Sun and Moon: when do they rise and set (Windows 9x),
also sundials for one day (northern hemisphere):
http://nest.sikornik.gliwice.pl/~piotrcf/eclip/sunr5.exe

Calendars (Windows 9x, Polish version):
http://nest.sikornik.gliwice.pl/~piotrcf/eclip/kalen.exe

Function Mul365(x:word):longint;
  inline(
          $58/              {POP     AX}
          $BB/$6d/1/        {MOV     BX,365}
          $F7/$E3           {MUL     BX}
         );

Function Div365(x:longint):word;
  inline(
          $58/              {POP     AX}
          $5A/              {POP     DX}
          $BB/$6d/1/        {MOV     BX,365}
          $F7/$F3           {DIV     BX}
         );

const Dbfmlut:array[boolean,1..12] of word=
         ((0,31,59,90,120,151,181,212,243,273,304,334),
          (0,31,60,91,121,152,182,213,244,274,305,335));

Function DateNro(y,m,d:integer):longint;
begin
  DateNro:=d+dbfmLut[(y and 3=0) and
           ((y mod 100>0) or (y mod 400=0)),m]
           +(y-1) div 4-(y-1) div 100+(y-1) div 400-1
           +mul365(y-1)
End;

{$b-}
Procedure Date(nro:longint; var y,m,d:integer);
var dl:integer;
    leap:boolean;
Begin
  y:=Div365(nro)+1;
  dl:=nro-mul365(y)-(y div 4-y div 100+y div 400);
  inc(y);
  repeat
     dec(y);
     leap:=(y and 3=0) and ((y mod 100>0) or (y mod 400=0));
     inc(dl,365+ord(leap));
  until dl>=0;

  m:=word(dl) div 32+1;
  if (m<12) and (dl>=Dbfmlut[leap,m+1]) then inc(m);
  d:=dl-dbfmlut[leap,m]+1;
End;

Function WeekDay(nro:longint):word; { 0: Su, 6: Sa }
begin
  Weekday:=(nro+1) mod 7;
End;

Thank You! These are going to be very useful.

Best Regards,

<URL: http://www.merlyn.demon.co.uk/programs/dateprox.pas> has code;
<URL: http://www.merlyn.demon.co.uk/programs/mjd_date.pas> tests it;
<URL: http://www.merlyn.demon.co.uk/programs/version.pas> needed;
TP7/BP7/D3 compatible.

The routines are sufficiently fast; but since they are programmed from
first principles in am attempt to be obviously correct, they may be a
little slower than those of others; in which case use them for
verification.

Converting "normal" date to JD (r: year, m: month, d: day):
===========================================

var dt: integer;   {a global variable: day of week }

function DataJD(r,m,d:real):real;
var a,b,c,e:real;
begin
  if r<0 then r:=r+1;
  a:=4716+r+int((m+9)/12);
  b:=1729279.5+367*r+int(275*m/9)-int(7*a/4)+d;
  c:=int((a+83)/100);
  e:=int(3*(c+1)/4);
  a:=b+38-e;
  if a<2299160.5 then a:=b;
  DataJD:=a;
  dt:=round(a+1.1-7*int((a+1.1)/7));
  while dt<0 do dt:=dt+7
end;

An auxiliary function (sorry, I don't remember the reason
why isn't it a simple int):

FUNCTION Int1(x:REAL):REAL;
BEGIN
  IF x>=0 THEN Int1:=int(x)
    ELSE
  IF x=int(x) THEN Int1:=x ELSE Int1:=int(x)-1
END;

Converting a JD to (year, month, day, hour):
===================================

PROCEDURE RokMD(jd: REAL; VAR r,m,d:INTEGER; VAR gr:REAL);
VAR w,x,u,z,y,a,b,c,e,f:REAL;
BEGIN
  gr:=jd-int1(jd-0.5)-0.5;
  w:=int(jd+0.5)+0.5; x:=int(w); u:=w-x;
  y:=int((x+32044.5)/36524.25); z:=x+y-int(y/4)-38;
  IF jd<2299160.5 THEN z:=x;
  a:=z+1524; b:=int((a-122.1)/365.25); c:=a-365*b-int(b/4);
  e:=int(c/30.61);
  f:=int(e/14);
  r:=round(b-4716+f); m:=round(e-1-12*f); d:=trunc(c+u-int(153*e/5)-0.5);
  IF r<=0 THEN r:=pred(r);
  dt:=round(jd+1.1-7*int((jd+1.1)/7));
  WHILE dt<0 DO dt:=dt+7
END;