php 날짜 관련해서 질문좀 드리겠습니다.

<?php
$TargetDate = $_POST["TargetDate"];
if($TargetDate == "")
{
    $TargetDate = date("Y-m-d");
}

$PreDay = date("Y-m-d", strtotime($TargetDate . " -1 day"));
$NextDay = date("Y-m-d", strtotime($TargetDate . " 1 day"));
?>


<form method="POST" id="PreDateForm">                    
<input type="hidden" name="TargetDate" id="TargetDate" value="<?php echo $PreDay;?>" />
<?php
        $PreCompare = date("Y-m-d", strtotime(date("Y-m-d") . " -3 day"));
        if($TargetDate > $PreCompare)
        {
echo "<a href=\"#\" onclick=\"document.getElementById('PreDateForm').submit(); return false;\">전날</a>";
}
else
{
echo "<a style=\"cursor:pointer;\" onclick=\"javascript:alert('3일 이전의 데이터는 확인이 불가능합니다.'); return false;\">전날</a>";
}
?>
</form>

<?php echo $TargetDate;?>