Introduction
WHMCS uses an internal configuration setting for max execution time and the following may be seen in WHMCS cron jobs.
PHP Fatal error: Maximum execution time of 300 seconds exceeded in /path/to/file.php
Procedure
Modify the WHMCS configuration.php file and add:
$overidephptimelimit = 500;
Please refer to WHMCS documentation at: Max Execution Time