You can use the day a trip starts or ends or the day a file is created, confirmed, or archived as the point of reference to calculate a task's due date.

However, certain options only appear when Before or After is selected in the task edit menu.

This is because it's impossible, for example, to schedule a task for before you confirm the file - because there is no way to know when you are going to confirm the trip. Safari Portal is good but we're not that good!