If field date is 15 days away from today, change another field value
Hmm.. This sounds like it could be done with Rules and VBO. 


Create a VBO view of nodes that match your date requirement using filter date field &gt; relative date. You might also want to filter out nodes that have already been changed.
Create a rule:


Event: Cron or maybe the Rules Once per Day module
Action: Load a list of entity IDs from a VBO view  
Action: Loop list  
Action: Change / Add field using Set a data value if it's a single value field or Add item to list if it's a multi-value field.



EDIT: As of November 2017, this feature whereby VBO supplies Rules has been a bit broken and requires a patch to work.