I was making a big mistake: I was checking a date field (data_scontrino_reg) against itself using:
<?php echo (date('Y-m-d',strtotime($_POST["data_scontrino_reg"],'-15 day'))); ?>
as MIN DATE.
So, it was always failing.
But I need to check it against the current date, so I used:
<?php echo (date('Y-m-d',strtotime('-15 day'))); ?>
and it works.
Sorry for making this confusion.