Task reminder Notify email not working

Though I have my email correctly entered in my user profile, the Notify Email of Task Reminder does not seem to work at all.

Is there anything that needs to be defined elsewhere?

Is there a way to test whether sending mail from this feature works properly?

Also, how can the time of sending be defined?

ie: If I need to call back my customer at 9pm next Friday, I need the system to send me a task reminder email at 8:45 for instance.

Since the task reminder form does only include a calender but no time scheduler, how can this be done?

Thanks for your help.