Inverse Julian Day Number Calculator

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

ENTER Signed JD Number

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                      For Signed Julian Day Number: 2461304

                Gregorian  Calendar Date  =  AD 02026-Sep-20-Sun
                Old Julian Calendar Date  =  AD 02026-Sep-07-Sun

         The DoW is the same for the same JD Number on either calendar.

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 signed JD numbers on 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 function is the inverse of the JD_Num() function and returns the
   calendar date string in two possible 'Ymd' string formats as:
   '-1949/5/7'  or  'BC 01949-May-07'

   J|G Calendar Mode: 'J' = Julian  or  'G' = Gregorian = Default

   FullDateFlag = FALSE = Ymd Format: '-1949/5/7' = Default format
                = TRUE  = Ymd Format: 'BC 01949-May-07-Tue'

   ERRORS:
   Returns FALSE on error = Non-numeric JD number or out of range JD number.
   ###########################################################################
*/

   function Inv_JD_Num ($JDNumber, $JorG='G', $FullDateFlag=FALSE)
{
   if (!Is_Numeric(trim($JDNumber))) {return FALSE;}

   $JDNum = floor(trim($JDNumber) + 0.5);
   $JorG  = substr(StrToUpper(trim($JorG)),0,1);
   $JorG  = ($JorG == 'J')? 'J':'G';
   $CAdj  = ($JorG == 'J')?   0:1;

   if ($JorG == 'J')
      {if ($JDNum < -5583576 or $JDNum > 9026423) {return FALSE;}}

   if ($JorG == 'G')
      {if ($JDNum < -5583424 or $JDNum > 9026275) {return FALSE;}}

  $A = floor($JDNum + 0.5);
  $B = $CAdj*floor(($A - 1867216.25) / 36524.25);
  $C = $A + $CAdj*($B - floor($B/4) + 1);
  $D = $C + 1524;
  $E = floor(($D - 122.1) / 365.25);
  $F = floor(365.25 * $E);
  $G = floor(($D - $F) / 30.6001);
  $d = $D - $F - floor(30.6001 * $G);     // Day num   (1 to 31)
  $m = $G - 12*floor($G/14) - 1;          // Month num (1 to 12)
  $y = $E - 4716 + floor((14 - $m) / 12); // Mathematical year (y)
  $Y = ($y > 0)? $y : $y-1;               // Calendar year (Y) -Neg = BC

  if ($FullDateFlag)
 {
  $AbsY   = SPrintF("%05d", abs($Y));
  $Y      = ($Y < 0)? "BC $AbsY" : "AD $AbsY";
  $d      = SPrintF("%02d", $d);
  $Mmm    = substr('JanFebMarAprMayJunJulAugSepOctNovDec', 3*($m-1), 3);
  $DoW    = (7 + ($JDNum + 1) % 7) % 7;
  return "$Y-$Mmm-$d-" . substr('SunMonTueWedThuFriSat', 3*$DoW, 3);
 }
  return "$Y/$m/$d";

} // END of  Inv_JD_Num()