将JS Dates（）传递给PHP会减少18个小时

Question

I want to select MySQLi records between a startdate and enddate so I have a JS script that allows me to enter dates (using HTML datebox type inputs) which I pass to a PHP script using JQuery's $.post method. My JS code is:

startDate = new Date(document.getElementById("fromDate").value);
startDate = new Date(startDate.getUTCFullYear(),startDate.getUTCMonth(), startDate.getUTCDate(), startDate.getUTCHours(), startDate.getUTCMinutes(), startDate.getUTCSeconds()); 
endDate = new Date(document.getElementById("toDate").value);
endDate = new Date(endDate.getUTCFullYear(), endDate.getUTCMonth(), endDate.getUTCDate(), endDate.getUTCHours(), endDate.getUTCMinutes(), endDate.getUTCSeconds()); 
$.post("DBQuery.php", {
    startdate: startDate,
    enddate: endDate,
},
function (output) {
var result = JSON.parse(output);
    $('#PHPStartDate').html(result.startdate).show();
    $('#PHPEndDate').html(result.enddate).show();
});

My PHP code is:

$startdate = date('Y-m-d h:i:s',strtotime($_POST["startdate"]));
$enddate = date('Y-m-d h:i:s',strtotime($_POST["enddate"]));
$con = mysqli_connect("localhost","username","password","dbase");
if (mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
$sql="SELECT * FROM Hits WHERE timestamp BETWEEN '$startdate' AND '$enddate'";
if ($result = mysqli_query($con,$sql)) {
$visitors = mysqli_num_rows($result);
}
$sql="SELECT DISTINCT session FROM Hits WHERE timestamp BETWEEN   '$startdate' AND '$enddate'";
if ($result = mysqli_query($con,$sql)) {
$sessions = mysqli_num_rows($result);
}
$sql="SELECT DISTINCT country FROM Hits WHERE timestamp BETWEEN '$startdate' AND '$enddate'";
if ($result = mysqli_query($con,$sql)) {
$countries = mysqli_num_rows($result);
}
$sql="SELECT DISTINCT city FROM Hits WHERE timestamp BETWEEN '$startdate' AND '$enddate'";
if ($result = mysqli_query($con,$sql)) {
$cities = mysqli_num_rows($result);
}
$output = json_encode(array("visitors"=>$visitors,"sessions"=>$sessions,"countries"=>$countries,"cities"=>$cities,"startdate"=>$startdate,"enddate"=>$enddate));
echo $output;
mysqli_free_result($result);
mysqli_close($con);

However the dates returned by PHP are 18 hours behind the dates which JS passed to PHP - very weird! My browser and MAMP server are on the same machine (my iMac) and I live in Thailand (UTC + 7 hours).

Here's a screen shot of what is displayed in HTML using the following code:

<div id="reportTitle">
            Report of site activity from
            <span id="startTime"></span>on<span id="startDate"></span>to
            <span id="endTime"></span>on<span id="endDate"></span><br>
            for<span id="showVisitors"></span>
            <span id="showCities"></span>
            <span id="showCountries"></span>
        </div>
        <div id="reportBody">
            <div>
                <span><h3>From</h3></span><span id="PHPStartDate"></span>
                <span><h3>to</h3></span><span id="PHPEndDate"></span>

The first set of dates displayed at the top of the screenshot are simply the JS startdate and enddatevariables and the lower set of dates are those returned by the PHP variables $startdate and $enddate which ought not to have changed (but in fact are 18 hours earlier!). Screenshot of JS and PHP dates I suspect the problem is to do with timezones but given that Thailand is UTC+7 it's hard to work out where the 18 hour time difference is coming from.