You can not change this setting with
ini_set() when running in safe mode.
The only workaround is to turn off
safe mode or by changing the time
limit in the php.ini.
— PHP manual, Runtime Configuration, description of max_execution_time
Many hosts run in safe mode. The other server is most likely one of them.
Edit: The same restriction is in place on set_time_limit:
This function has no effect when PHP
is running in safe mode. There is no
workaround other than turning off safe
mode or changing the time limit in the
php.ini.
— PHP manual, set_time_limit page