Signed Julian Day Number Calculator

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

Era

Year #

Month #

Day #

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                  Calendar Date: Yyyyy/mm/dd = 02026/05/03

               Julian Calendar JD Number = 2461177	DoW = Sat
            Gregorian Calendar JD Number = 2461164	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 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
Where DoW : 0=Sun, 1=Mon, 2=Tue, 3=Wed, 4=Thu, 5=Fri, 6=Sat
Applies equally to either calendar system.

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/*
   This PHP function computes the signed Julian Day Number on the old
   Julian or the modern Gregorian calendar system.

   It also handles proleptic dates prior to the mathematical origin of
   either calendar system and can handle negative Julian Day numbers
   outside the range of the native built-in PHP JD number functions.

   This generic function does NOT check for invalid dates except to see
   if the ('Yyyyy/mm/dd') date arguments are all numeric values.

   ARGUMENTS:
   Input argument strings use the format:
   YmdString = 'Yyyyy/mm/dd'  (Examples: '-01949/05/20' or '1949/5/20')

   Yyyyy = Signed calendar year number <> 0
           -Negative = BC  and  +Positive = AD

       Span: BC 20000 to AD 20000
       There is no calendar year 0 (zero).

   m = Month number (1 to 12).
   d = Day number   (1 to 31).

   JorG = Calendar mode switch.
          'J' = Julian calendar
       or 'G' = Gregorian calendar = Default

   ERRORS:
   Returns error message if any of the ('Yyyyy/mm/dd')
   arguments are non-numeric.

   NO EXTERNAL DEPENDENCIES
*/

   function JD_Number ($YmdString, $JorG='G')
{
   $YmdString = trim($YmdString);
   list($Y,$m,$d) = PReg_Split("[\/]", $YmdString);
   $Y = trim($Y);   $m = trim($m);     $d = trim($d);

// Error if any of the (Y, m, d) elements are non-numeric.
   if (!Is_Numeric($Y) or !Is_Numeric($m) or !Is_Numeric($d))
      {return "ERROR: Non-Numeric Date Element(s).";}

// Get calendar mode argument (Default = 'G').
   $JorG = (StrToUpper(substr(trim($JorG),0,1)) == 'J')? 'J' : 'G';

// Compute JD number according to the Julian calendar.
   $A     = floor((14-$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 the computation is for the Julian calendar,  then we
// are done.  Return the JD number for the Julian calendar.
   if ($JorG == 'J') {return $JDNum;}

/* Otherwise,  apply this adjustment  to convert the  JD number on the
   old Julian calendar into  the JD number for our Gregorian calendar.
   This adjustment may  equate to a negative  or positive value and is
   the difference in days between the two calendar systems on the same
   given date in the context: DiffDays = (GregJDNum − JulJDNum).
*/
   $w = floor(((($Y < 0)? $Y+1 : $Y) - floor((14-$m)/12)) / 100);
   $JDNum += (floor($w/4) - $w + 2);

   return $JDNum;

} // END of  JD_Number (...)