What Ken said +1.
The code is currently:
$stop_timer = script_time(); // stop script time
$tScript = ''; // set variable
$tScript += ($stop_timer - $start_timer); // get start & stop time
$time_cull = sprintf("%01.4f", round($stop_timer - $start_timer, 4)); // calculate difference
$noted .= "<!-- Data process time: $time_cull seconds -->\n"; // display time results
$tscript therefore is a string, which creates the error. Actually, you don't even need that line if you get rid of the plus in the assignment:
$tScript = ($stop_timer - $start_timer);