due date default to n days from now in migration

hello,how can I set the default value of a column in the migration
called due_on with type datetime to n days from now at the time when a
model is created ?

Thanks Ar Chron, - one question: why after create is chosen over
before create in supplying the date that's n days from now? Could it
be that this is for validation concerns?

Sorry I was out of internet for a few days. -- I see, so this is so
that some sort of predetermined/ non-empty date will always be
supplied in a business logic point of view.