PHP：在特定时间发布推文

I'm building an application on WordPress wherein I have a single form with two entries:

1) content of the tweet

2) time at which the tweet should be posted on the user's Twitter timeline

Form:

<form method = "POST">

<input type = "text" name = "tweet1">
<input type = "datetime-local" name = "date1">
<input type = "submit">

</form>

Now, on submitting the form, I want to post the content of the text field to be posted on the user's Twitter timeline at the time specified by the user in the form. $_SESSION['x'] and $_SESSION['y'] stores the user's oauth_token and oauth_token_secret respectively.

<?php
    if(isset($_POST['submitted']))
    {   
        function do_this_one()
        {
            $connection = new TwitterOAuth(CONSUMER_KEY, CONSUMER_SECRET, $_SESSION['x'], $_SESSION['y']);
            $response = $connection->post("statuses/update", array('status' => $_POST['tweet1']));
        }
        date_default_timezone_set('Asia/Calcutta');
        $myDate = strtotime($_POST['date1']);
        $now = time();
        $ti_one = $myDate - $now;
        echo time() + $ti_one;
        add_action( 'my_new_event','do_this_one' );
        wp_schedule_single_event(time() + $ti_one, 'my_new_event' ); 
?>

$ti_one variable basically stores the time in seconds between now and the time specified by the user. For example, if the current time is 2:00 PM, then if the time specified by the user in the form is 2:03 PM, then $ti_one would be equal to 180 seconds. That's the reason why I'm scheduling the event at time() + $ti_one in:

wp_schedule_single_event(time() + $ti_one, 'my_new_event' );

However, on submitting the form, the tweet is not being posted at the specified time. The above code ie code for the form and the PHP code is inside callback.php, which is the page that the users are redirected to after granting permission to the OAuth client.

What seems to be wrong with my code? Am I scheduling the event incorrectly?