WP_Query filter Posts by timestamp event (range start and end) and by month (next 12 month)

Resolved.
My error are there :

$FirstDayThisMonth[] = date('Y-m-01 00:00:01', mktime(0, 0, 0, $x, 1));           
$LastDayThisMonth[] = date('Y-m-t 23:59:59', mktime(0, 0, 0, $x, 1));

instead of :

$datestart = strtotime(date('Y-'.$nbofmonth.'-01 00:00:01'));   
$dateend = strtotime(date('Y-'.$nbofmonth.'-t 23:59:59')); 

It’s always very annoying to have well written code and to realize that a small element breaks everything