I am using a Process builder to set value to the field expiry_date__c based on booking_date__c
Example 1:
If booking_date__c is 12th June 2017, the expiry_date__c must be calculated as current month + 3 full months from July. So expiry date must get populated to 30th September.
Example 2: If booking_date__c is 5th February 2017, expiry_date__c must be 31st May 2017