PHP
add_action('wp_footer',function(){
$show = date_diff(date_create("2022-12-02"), date_create())->invert;
if($show){
echo <<<EOF
<style type="text/css">
html {
-webkit-filter: grayscale(100%);
-moz-filter: grayscale(100%);
-ms-filter: grayscale(100%);
-o-filter: grayscale(100%);
filter: progid:DXImageTransform.Microsoft.BasicImage(grayscale=1);
_filter: none;
}
</style>
EOF;
}
});