[Fixed]-Django cron-job for user-specified times

Since you tagged your question with celery, I assume you have celery running. You could use the eta kwarg to apply_async() to schedule a task to run at a specific time, see here:

http://docs.celeryproject.org/en/latest/userguide/calling.html#eta-and-countdown

If you need to use a cron job, I would not check if notification_time == current_time, but rather track unsent notifications with a boolean is_sent field on the model and check for notification_time <= current_time and not is_sent. This seems to be slightly less error prone. You could also add some form of check to prevent mass-sending notifications in case your system goes down for a few hours.