In a Deal I have a date that I would like to set a default value for based on another field.

For sake of the example, I'll call the first "Arrival" and the second "Cut-off". I want the filed "Cut-off" to be defaulted to three weeks before "Arrival". (Example: Arrival is set to 30-Dec-2018, Cut-off is defaulted to 9-Dec-2018)

Under Workflows, I can setup the triggering event when the Arrival field is changed, however the Field Update does not allow me to select another field as the base to "minus" the 21 days.

Will I need to write a custom function and if so, can you give me some assistance?

Thanks,

Trey Noe