function GregorianToJD ($month,$day,$year) { 
    if ($month > 2) { 
        $month = $month - 3; 
    } else { 
        $month = $month + 9; 
        $year = $year - 1; 
    } 
    $c = floor($year / 100); 
    $ya = $year - (100 * $c); 
    $j = floor((146097 * $c) / 4); 
    $j += floor((1461 * $ya)/4); 
    $j += floor(((153 * $month) + 2) / 5); 
    $j += $day + 1721119; 
    return $j; 
}

function JDToGregorian($julian) {
    //return planner123_widget_main::fn_calcJDToGregorian($julian);
    $julian = $julian - 1721119;
    $calc1 = 4 * $julian - 1;
    $year = floor($calc1 / 146097);
    $julian = floor($calc1 - 146097 * $year);
    $day = floor($julian / 4);
    $calc2 = 4 * $day + 3;
    $julian = floor($calc2 / 1461);
    $day = $calc2 - 1461 * $julian;
    $day = floor(($day + 4) / 4);
    $calc3 = 5 * $day - 3;
    $month = floor($calc3 / 153);
    $day = $calc3 - 153 * $month;
    $day = floor(($day + 5) / 5);
    $year = 100 * $year + $julian;
    
    if ($month < 10) {
        $month = $month + 3;
    }else{
        $month = $month - 9;
        $year = $year + 1;
    }
    
    if ($day < 10) {
        $day = "0".$day;
    }
    
    if ($month < 10) {
        $month = "0".$month;
    }
    
    //return $day."/".$month."/".$year;
    return $month."/".$day."/".$year;
}

줄리안(Julian) -> 그레고리안(Gregorian) 변환 (버전2)

function fn_calcJDToGregorian($julian) {
    $z = $julian;
    $f =  .5;
    $f = 0.0; // PHP함수와 결과값 맞추기 위해 0.5일 조정함

    if($z < 2299161){
        $a = $z;
    }else{
        $alpha = floor(($z - 1867216.25) / 36524.25);
        $a = $z + 1 + $alpha - floor($alpha / 4);
    }

    $b = $a + 1524;
    $c = floor( ($b - 122.1) / 365.25);
    $d = floor( 365.25 * $c);
    $e = floor( ($b - $d) / 30.6001);
    $dd = floor( $b - $d - floor(30.6001 * $e) + $f);

    if($e < 13.5) {
        $mm = $e - 1;
    }else{
        $mm = $e - 13;
    }

    if($mm < 2.5 ){
        $yy = $c - 4715;
    }else{
        $yy = $c - 4716;
    }

    return "$mm/$dd/$yy";
}

[출처] http://flashcafe.org/php_function/8705