In this video, I walk you through how to set a due date X number of days before a date that is stored in a custom date field.
This formula I used to set the due date in this example is DATEADD(cf_date_of_departure,-70)
DATEADD( ) is a function that adds some number of days to a date
cf_date_of_departure refers to the custom field that stores our date of departure. In our field settings, we set the field key to date_of_departure. When we apply it to a formulas we add cf_ as a prefix to end up with cf_date_of_departure.
-70 is the number of days we are adding. Since it is a negative number, we are subtracting days to end up with 70 days prior to the date of departure.