You are using a trigger of "before page loads" but referring to a posted form value... the form post won't exist before the page loads because it won't have been posted.
If you want to set the value before the page loads you should be binding it directly to:
<?php echo (date("Y-m-d G:i:s"));?>
and you can add the line:
<?php date_default_timezone_set($row_CompanyDetails['TimeZone']); ?>
right above it on line 223
You can't refer to a form post in the before page loads triggered action because no form is being posted when the page loads.