One possible option is that the administrator may have installed another task management program such as atd, which can be used to do regular jobs just like cron: http://linux.die.net/man/1/at
Or the system administrator may have simply locked down cron to stop you from using it. But you can still install cron separately onto your own user account. Warning: I've not tried this and it may not work. Basically, download the tarball, and install it using
./configure --prefix=/home/username/cron && make && make install
(where cron is a directory you have created). You should then be able to run
(possibly usr/bin or just bin/ or sbin/). Don't forget to also run cron itself.
The simplest option, if you can't speak to whoever runs the server, is to write your own script to poll the time periodically and run your job whenever it needs to be run.