[CFMaBiSmAd]

Here is a script that does this (tested with various time zones, including some of the 1/2 hour and 3/4 hour ones) -

PHP Code:

<?php
if (isset($_GET['offset'])) {
    $minutes = $_GET['offset'];
    echo "GMT offset (in minutes, from the browser): ". $minutes ."<br />\n";
    echo "GMT: ". gmdate("Y-m-d H:i:s") ."<br />\n";
    
    $local = gmmktime(gmdate("H"),gmdate("i")-$minutes); // adjust GMT by client's offset
    
    echo "Calculated client's date/time: ". gmdate("Y-m-d h:i:s a",$local) ."<br />\n";
} else {
    echo "<script language='javascript'>\n";
    echo "var d = new Date();\n";
    echo "location.href=\"${_SERVER['SCRIPT_NAME']}?offset=\" + d.getTimezoneOffset();\n";
    echo "</script>\n";
    exit();
}
?>