You can always create a task to run on or before that date. It will generate a ticket in the system which will in turn send out an email. If you go to Setup> Tickets> Tasks and create a new task. For the Subject line put "Warranty" and the details of the warranty in the Request Details area. You can schedule it to the date you would like how ever many months from now the task needs to launch. Once the task is launched you can edit the schedule for the next round of warranties or delete the task. I hope this helps.