PHP - 在开始时强制使用session_start

Created a simple time function which it returns a foreign time when user clicks on Now. Initially it didn't require session_start, but after a few test runs it started to give this error.

php

$start_time = $_GET['time'];
$city = $_GET['city'];
date_default_timezone_set(city);
echo date('Y-m-d H:i:s', strtotime($start_time));

What I understand about session_start is it is needed for session variables to work, but this situation has nothing to do with session, it is just a simple script. So I hope someone can enlighten me please? Thanks in advance