I think I haven't posted my question the right way.
I need to validate the date inserted checking if it is more than 15 days old.
today is 10 may 2017.
If the user enters the date 10 april 2017: date invalid;
If the user enters the date 5 may 2017: date valid.
I tried this:
<?php echo (date('Y-m-d',strtotime($_POST["data_scontrino_reg"],'+15 day'))); ?>
but it doesn't work.