It would be great if there was a date of activity constraint of 'xx time ago', eg 2 days ago.
As an example, if you want to set up a trigger campaign that fires a given amount of time after a certain event happens (say you want to send an email 2 days after someone has filled out a form), you need to set the trigger in your smart list, and then set a wait step in the flow of 2 days.
It would be a lot easier if there was a date constraint that allowed you to select this time frame within the trigger, so your trigger would be 'Fills out form: Date of activity = 2 days ago'