PHP中的DateTime返回错误的值？

First off, i have been struggling for two days non stop to figure out what I need to with this code and still stuck in the square one.

so here I am again and i hope someone shed a light on this issue as my head is exploding.

what I need to do is to get the difference between two values using DateTime in PHP.

The first value is 00:00 which is a set value and never gonna change.

the second value is $offset/3600 * 1 which is a time difference between two timezones.

the code I am using is this:

<?php
if (0 > $offset)
{
// set an object with the current date
$date = new DateTime();
$date->setTime(00, 00);

// the second date
$date2 = new DateTime($offset/3600 * 1);

// apply the diff() method, getting a DateInterval object ($diDiff)
$diDiff = $date->diff($date2) ;
}
echo $diDiff->format("H:i");
?> 

the code above echo's this: H:i and nothing else!

what am i doing wrong?

Thanks in advance.

EDIT:

The $offset comes from here:

if( isset($_POST['submit']))
{
    //be sure to validate and clean your variables
    $timezone1 = htmlentities($_POST['timezone1']);
    $timezone2 = htmlentities($_POST['timezone2']);

    //then you can use them in a PHP function. 
    function get_timezone_offset( $origin_tz, $remote_tz ) {
    $timezone1 = new DateTimeZone( $origin_tz );
    $timezone2 = new DateTimeZone( $remote_tz );

    $datetime1 = new DateTime("now", $timezone1);
    $datetime2 = new DateTime("now", $timezone2);

    $offset = $timezone1->getOffset($datetime1) - $timezone2->getOffset($datetime2);
    return $offset;

}

$offset = get_timezone_offset($timezone1, $timezone2);

}

?>

timezone1 and timezone2 are two dropdown lists with php timezones inside them and they are identical.

like so:

  <select name="timezone2" id="timezone2" class="timezone2">
                        <?php
                    foreach($options as $key => $value)
                    {
                        echo '<option value="'. $key .'" label="'. $value .'">'.$value.'</option>';
                    }
                    ?>
<option value="Africa/Abidjan" label="Abidjan">Abidjan</option>
<option value="Africa/Accra" label="Accra">Accra</option>
<option value="Africa/Addis_Ababa" label="Addis Ababa">Addis Ababa</option>
<option value="Africa/Algiers" label="Algiers">Algiers</option>
<option value="Africa/Asmara" label="Asmara">Asmara</option>

</select>

With the DateInterval object, you have to prefix all the formatting letters with the "%" character. So use:

echo $diDiff->format("%H:%i");

Happy hunting!