Calculating time Difference with Submission Pre-Parser

[ramonbps]

So i been trying to get this going calculating time difference of my for form.

Below is my code.

if (isset($_POST["checkin"]) && $_POST["checkout"]) {
$tcheckin = $_post['checkin']; $tcheckout = $_post['checkout'];
 $tdiff = date_diff("$tcheckin","$tcheckout")->format("%H:%I:%S");
$_POST["inandout"]=["$tdiff"];
exit;

I am not sure what i am doing wrong.

 this is my actually code

PHP Code:

$tcheckin=$_POST["checkin"];
$tcheckout=$_POST["checkout"];
$tdiff = date_diff($tcheckin,$tcheckout)->format("%H:%I:%S");
$_POST["inandout"]=["$tdiff"];

When i remove ->format("%H:%i:%s") the blank page goes away. 


Attach is the blank page i get.