Ho scritto il codice in questo modo, come del resto da te suggerito, adesso mi compare l'alert "SONO QUI", ma il calendario comunque non fa quello che deve fare o meglio permette comunque di selezionare qualsiasi data:
PHP:
<script type="text/javascript"></script>
<?php include("js/cavicchi.js"); ?>
<?php
$anno = date("Y",strtotime("+4 days"));
$mese = date("m",strtotime("+4 days"))-1;
$giorno = date("j",strtotime("+4 days"));
?>
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>jQuery UI Datepicker - Default functionality</title>
<link rel="stylesheet" href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
<link rel="stylesheet" href="/resources/demos/style.css">
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
<script type="text/javascript">
$(document).ready(function(){
alert('SONO QUI ! '+new Date(<?php echo $anno;?>, <?php echo $mese;?>, <?php echo $giorno;?>));
/* JS PER CALENDARIO DATA PICKER */
$(function(){
$( "#datepicker" ).datepicker();
$.datepicker.setDefaults( $.datepicker.regional[ "it" ] );
});
$(function(){
$( "#datepicker1" ).datepicker();
$.datepicker1.setDefaults( $.datepicker.regional[ "it" ] );
});
});
</script>
</head>
<body>
<form class="form" name="inscomm" method="post" id="modulo" action="index.php?page=salva_commessa" enctype="multipart/form-data">
<div class="form-container">
<div class="card-commesse">
<div class="card-header">Anagrafica Cliente</div>
<div class="card-block">
<div class="row">
<div class="col-md-2">
<div class="input-group">
<span class="input-group-addon"><i class="far fa-calendar-alt" aria-hidden="true"></i></span>
<input type="text" class="form-control" id="datepicker" placeholder="Data Commessa" name="data" autocomplete="off" required>
</div>
</div>
<div class="col-md-2">
<div class="input-group">
<span class="input-group-addon"><i class="far fa-calendar-alt" aria-hidden="true"></i></span>
<input type="text" class="form-control" id="datepicker1" placeholder="Scadenza Revisione" name="dtscrev" autocomplete="off">
</div>
</div>
</div>
ecc. ecc.
</div><!--Close Form Container-->
</form><!--Close Form-->
</body>
</html>