Business Rule based on field change in a custom object

Summary

I want to initiate a rule when a custom field has reached a certain date

Content

Rule Types are Incident, Answer, Contact, Organisation, Task, Opportunity, Offer Advisor and Chat Rule Types.

I have some custom fields in a custom object how can I incorporate these into my business rule.

Is it possible, for example to have the custom objects added to the list of Rule Types so they can be selected?

What I want it to do when it has checked the date is to send a reminder email to the user which seems easier with Opportunity.

Does anyone have any examples of similar functionality. Thanks in advance.