I was playing around and wondered how accurate would it be to use a negative unix timestamp. Well, it appears to be pretty accurate.

Check it out, here’s a generic function.

function backdate($time){

    $day = 86400;
    $year = 31536000;

    if($time <= 0){
        $timeleft = $time;
        $yy = (floor($timeleft/$year))*-1;
        $timeleft = $timeleft + ($yy*$year);
        $dd = floor($timeleft/$day);
        $year = "19".(70-$yy);
        $yyy = intval((72-(70-$yy))/4);

    }else{
        $timeleft = $time;
        $yy = intval($timeleft/$year);
        $timeleft = $timeleft - ($yy*$year);
        $dd = intval($timeleft/$day);
        $yyy = (intval($yy/4)*-1)+1;
        $x = 70+$yy;
        if($x > 99){
            $x = $x - 100;
            if($x<=9){$x="0".$x;}
            $year = "20".$x;
        }else{
            $year = "19".(70+$yy);
        }

    }

    $yyy = $yyy;
    $dd = $dd+$yyy;

    if($dd > 365){
        $year = $year+1;
        $dd = $dd - 365;

    }

    if($dd < 1){
        $year = $year-1;
        $dd = 365+$dd;
    }

    if($dd >= 1 && $dd <= 31){
        $mm = 1;
    }else if($dd >= 32 && $dd <= 59){
        $mm = 2;
        $dd = $dd - 31;
    }else if($dd >= 60 && $dd <= 90){
        $mm = 3;
        $dd = $dd - 59;
    }else if($dd >= 91 && $dd <= 120){
        $mm = 4;
        $dd = $dd - 90;
    }else if($dd >= 121 && $dd <= 151){
        $mm = 5;
        $dd = $dd - 120;
    }else if($dd >= 152 && $dd <= 181){
        $mm = 6;
        $dd = $dd - 151;
    }else if($dd >= 182 && $dd <= 212){
        $mm = 7;
        $dd = $dd - 181;
    }else if($dd >= 213 && $dd <= 243){
        $mm = 8;
        $dd = $dd - 212;
    }else if($dd >= 244 && $dd <= 273){
        $mm = 9;
        $dd = $dd - 243;
    }else if($dd >= 274 && $dd <= 304){
        $mm = 10;
        $dd = $dd - 273;
    }else if($dd >= 305 && $dd <= 334){
        $mm = 11;
        $dd = $dd - 304;
    }else if($dd >= 335 && $dd <= 365){
        $mm = 12;
        $dd = $dd - 334;
    }else{
        $mm = "MM";
        //$dd = "DD";
    }

    if($dd <= 9 && $dd >= 1){
        $dd = "0" . $dd;
    }
    if($mm <= 9 && $mm >= 1){
        $mm = "0" . $mm;
    }

    return $mm . "/" . $dd . "/" . $year;

}

$input = "12/29/1951"; //input your date here
$parts = explode("/", $input);
$unix = mktime(0,0,0,$parts[0],$parts[1],$parts[2]);

echo "Input date is: " . $input;
echo "Unix convertion: " . $unix;
$convo = backdate($unix);
echo "The rebuilt time is: ". $convo;

I haven’t like crazy checked this out, but I did compare it against a db with over 200+ birthdays in it, and it appeared to be pretty accurate. Only flaw if any is the leap year adjustment.