Signed Julian Day Number Calculator

Long-Term Signed Julian Day Number Calculator
Span: BC 20000 to AD 20000
PHP Program by Jay Tanner - 2026

Calendar Date (Era, Yyyyy, mm, dd)

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                 Calendar Date: BC|AD Yyyyy/mm/dd = 02026/07/13

               Gregorian Calendar JD Number = 2461235	DoW = Mon
              Old Julian Calendar JD Number = 2461248	DoW = Sun
               Calendar Difference  (G − J) = -13 days

The Julian Day Number numerically equates to the Julian Date at 12:00:00 Noon
and is always a signed integer value.

Mathematical Calendar Origins:
BC 4713-Jan-01-Mon 12:00:00 = Origin date of Old Julian Calendar    JDNum = 0
BC 4714-Nov-24-Mon 12:00:00 = Origin date of  Gregorian Calendar    JDNum = 0

Julian Day Number  =  floor(Julian Date + 0.5)

For the calendar day of the week index (DoW = 0 to 6):
DoW = (7 + (JDNum + 1) mod 7) mod 7
Applies equally to either calendar system.

Where for DoW : 0=Sun, 1=Mon, 2=Tue, 3=Wed, 4=Thu, 5=Fri and 6=Sat

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/*
   ###########################################################################
   This PHP function computes the signed Julian Day Number on the old Julian
   or the modern Gregorian calendar system and can handle negative Julian Day
   numbers outside the range of the native built-in PHP JD number functions.
   Negative years represent BC years.

   NOTE: Year values can range from 1 to 5 digits long.

   Valid Example Arguments: 'sYyyyy/mm/dd'
   YmdStr = '-18191/05/25' or '1999/01/23' or '+17652/12/09' or '1/2/3'
*/
   function JD_Num ($YmdString, $JorG='G')
{
   $YmdStr = trim($YmdString);
   list($Y,$m,$d) = PReg_Split("[\/]", $YmdStr);
   $Y = trim($Y);   $m = trim($m);     $d = trim($d);

   if (!Is_Numeric($Y) or !Is_Numeric($m) or !Is_Numeric($d))
      {return FALSE;}

   $JorG  = (StrToUpper(substr(trim($JorG),0,1)) == 'J')? 'J' : 'G';
   $A     = floor((14-$m) / 12);
   $B     = (($Y < 0)? $Y+1 : $Y) - $A;
   $C     = floor($B/100);
   $JDNum = floor(30.6001*(12*$A + $m+1))+floor(365.25*($B+4716))-1524 + $d;

   if ($JorG == 'J') {return $JDNum;}

   $w = floor(((($Y < 0)? $Y+1 : $Y) - floor((14-$m)/12)) / 100);
   $JDNum += (floor($w/4) - $w + 2);

   return $JDNum;

} // END of  JD_Num (...)