function showUser() {
    // Retrieve values from the selects
    var DateFrom = document.getElementById('DateFrom').value;
    var DateTo = document.getElementById('DateTo').value;


    if (dtd=="" || dtm == "" || dfd == "" || dfm == "") {
        document.getElementById("txtHint").innerHTML="";
        return;
    }

    if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp=new XMLHttpRequest();
    } else {// code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }

    xmlhttp.onreadystatechange=function() {
        if (xmlhttp.readyState==4 && xmlhttp.status==200) {
            document.getElementById("txtHint").innerHTML=xmlhttp.responseText;
        }
    }

    xmlhttp.open("GET","/StreetHailDrivers.php?DateFrom="+DateFrom+"&DateTo="+DateTo,true);
    xmlhttp.send();
}

$(function() {
    $('.datepicker').datepicker({ onSelect: showUser(), minDate: -90, maxDate: "+1D" });
  });
  
  
  
  
  $("#datepicker").datepicker({

    onSelect: function(date, instance) {

        $.ajax
        ({
              type: "Post",
              url: "www.example.com",
              data: "date="+date
              success: function(result)
              {
                  //do something
              }
         });  
     }
});



http://jsfiddle.net/5BbGS/502/


http://jquerytutorialwithexamples.com/2014/11/jquery-datepicker-ajax-request-select-date/