Sending parameter as string or as date in JS

Question

I am trying to send a parameter in an AJAX call. I need to send it in a way that the receiver file can understand it as a date to be part of a mysql query.

The page receives the value as URL parameter, like this:

http://.....php?dia=2016-08-15

Then on this page I make an AJAX call to send the param as POST data, like this:

var loadDataJRZ = function(){
    $.ajax({
        type: "POST",
        data: {dia: "<?php echo $_GET['dia']?>"},

And then in the receiver file I should get the param like this: $dia = $_POST['dia'];

If I execute the receiver file directly in the browser, the output is ok. But running the web page, the output is an empty string.

This is the receiver file code:

$dia = $_POST['dia'];

$sql = "SELECT *
        FROM dbo.LoteJuarez 
        WHERE  fecha = '$dia'  
        ORDER BY fecha, hora DESC";

I guess it is a matter of variable type, then I get the value dia = 903 if I send the AJAX call without "", like this

 data: {dia: <?php echo $_GET['dia']?>},

and I get dia="2016-08-15" sending the call with ""

data: {dia: "<?php echo $_GET['dia']?>"},

Taking into account that a direct execution from the receiver file shows the query result as it should be, what should I change to make it work?